I was dealing with this issue lately so I decided to ask for some help.
The problem is that my PC is freezing whenever I play a game and I have to reboot my system.It takes like 3 to 30 seconds to freeze after I open the game (any game)
My specs:
*ASRock K7S41GX Motherboard
*AMD Athlon(tm) XP 2100+ Processor
*1GB SDRAM Memory
*On-board C-Media AC97 Audio Device
*On-board SiS 761 Video Device (not used)
*NVIDIA GeForce4 MX 4000 Video Device

My operating system Windows XP Professional SP2

Things I tried:
*Update Video/Audio Devices
*Use Anti-virus/Anti-spyware Software
*Update Windows
*Reinstall Windows(2 days ago)
*Search in D-A-L Forums for similar issues

Also:
*The Video card is not faulty, I swapped it with the On-board one and an ATI one and it`s the same story.
*Before I updated my Audio Device , I was getting an error (the Send/Don`t Send one) pointing to my Audio Device after rebooting because of a freeze, but now I don`t get it anymore.
*I had this issue in the past(like 3 months ago) and the Power Supply was the cause of the problem.I bought a new one and it solved it.Now,after buying another one (thinking the Power Supply can be the cause of the problem again) it solved nothing.
*The PC sometimes freezes with a BSOD but it restarts before even reading what the BSOD has to say.The only thing I could see is that it was a 0x0D1 error.

If any additional information is needed,I will add it as soon as possible.

Rebooting can be caused by many things - the first I always suspect and try to eliminate is heat - this is especially true when the problem surfaces during gaming, a very demanding task. Is your system clean of heat trapping dust and dirt? Do all fans spin freely? Are your temperatures fine?

To stop the computer from automatically rebooting, do the following:

1. Right-click My Computer, then click Properties > Advanced Tab,
2. Under Startup and Recovery, click Settings,
3. Clear the check in the Automatically restart check box,
4. Click OK as needed to complete and exit
5. Restart computer.

Now when you get a BSOD, it will freeze on that page so you can read the error. You can then force a reboot by pressing the reset button.

Inspect the motherboard for bulging or leaking electrolytic capacitors. These failed or failing capacitors are a common cause of sudden, but seemingly random system lock ups and reboots. The capacitors look like tall soda cans, many of which surround the CPU socket.

All older motherboards, and many of today's less expensive motherboards use electrolytic capacitors containing a liquid electrolyte. Failing (including flawed and/or abused/over-heated) capacitors literally bulge at the seams due to excessive internal pressures. Extreme (and very rare) cases result in a firecracker type explosion that can really stink up a room. Typically, electrolyte just oozes from the pressure relief point, which appears as a symbol or letter stamped in the top of the capacitor casing. The electrolyte can be caustic to motherboards and flesh. Look for white to dark-brown, dried liquid or foam on the tops or bottoms of the capacitors. Bulging capacitors are a sign leakage is about to occur.

A motherboard with bulging or leaky capacitors can be repaired, but often it is more cost effective in the long run to replace the motherboard.

Be sure to first power down, unplug the computer, and keep yourself discharged by touching the bare metal of the case before reaching in.

*Is your system clean of heat trapping dust and dirt? Do all fans spin freely? Are your temperatures fine?

First of all,I have my case open so my temperatures should be fine.By the looks of a software I use to measure the temperature,I have 40 degrees Celcius (104 degrees Fahrenheit) while the PC is idle.
The system is pretty clean but there is a little dust so I`ll try cleaning it tomorrow.
As for the fans they all spin freely.

*Automatically restart check box
This feature is unchecked by default on my operating system.

*BSOD
I know it shall freeze on the blue page,but this does not happen.
It only freezes on the blue page for a couple of seconds and then restarts automatically without having me press the Restart button.
This is very rare to happen because most of the time it just freezes on the last image shown, but if I get a BSOD lasting enough for me to write down the parameters, I will post them here.

*Motherboard
There is no smell coming from the motherboard and I see no bulging capacitors.All my capacitors are flat.

Hmmm, well you already replaced the power supply, twice, so that's not it. 40°C is fine for the CPU, but other devices, if over heated, can cause problems too. These include RAM, the chipset, and the graphics solution. Running with the case open is not always a good idea because a good case is designed to allow for front-to-back air flow. With the side open, flow is disrupted, and heat sits surrounding the component. When checking for heat issues, I generally recommend removing the side and blasting a desk fan in there (cleaning first, for obvious reasons).

Just to be sure the system is malware free, you might try purging the hard drives of clutter with Windows (XP or Vista) Disk Cleanup, ATF Cleaner or CCleaner. If using CCleaner, uncheck the option to install the Yahoo toolbar during installation. Before first use, go to Options > Settings > Advanced and ensure Only delete files in Windows Temp folders older than 48 hours is unchecked. Know your site credentials (user names and passwords) for sites you frequent before cleaning; you may have to login again at next visit.

Download and install Malwarebytes's Anti-Malware (MBAM) and scan as follows:

Check for updates before scanning,
Select Perform quick scan > Scan,
When complete, click OK, then Show Results,
Ensure all is selected, and click Remove Selected,
When complete, save the log in Notepad to a convenient place for future reference.

You might try testing the RAM using one of the following programs. Both require you to create and boot to a bootable floppy disk or CD to run the diagnostics. Using the floppy method is generally easier (and another reason to include floppy drives in new builds). However, the CD method is just as effective at detecting RAM problems. Allow the diagnostics to run for several passes or even overnight. You should have no reported errors.

Windows Memory Diagnostic - see the easy to follow instructions under Quick Start Information.
or
MemTest86+ (for more advanced users) - an excellent how-to guide is available here.

Alternatively, you could install a single RAM module and try running with that to see if it fails. Repeat process with remaining modules, hopefully identifying the bad stick through a process of elimination.

Ok so I downloaded the software you told me to.
*CCleaner
I runned the Cleaner and the Registry Fixer and fixed all the issues
* Malwarebytes's Anti-Malware
I runned it, found two Infected Objects -> Removed
*MemTest86
Followed the guide step-by-step and got it to run with a CD.
I let it to get to 100% Pass three times and the results are :
Pass : 3
Error: 0
Is there a certain number of times it has to pass?I got it to 3 times because I was away and let it run.
Even after I got after all those phases, I still encounter the freezing problem.

Well, I didn't say to run the Registry fix. Though CCleaner's Registry cleaner is about the only one I trust, all cleaners pose a risk, so I am glad it went fine.

3 is probably fine.

By freeze, you mean it just locks up? Any blue screens? Any errors in Event Viewer?

Yes,with "freeze" i mean "lock-up" but a permanent one.
The Event Viewer has no errors, just 2-3 warnings.
I got a BSOD like 5 minutes ago but not when I was in-game and it lasted enough to write it down :
0x00000050 ( 0xA0000000, 0x00000000, 0xA0000000, 0x00000000 )
but I`m not sure this is correct because some "0"s looked more like "o"s so it may be
0x00000050 ( 0xA0o0o000, 0x00000000, 0xAo0o00o0, 0x00000000 )
I did not wrote the "o"s in capital so they are easier to distinguish.

EDIT: Seems like I got another one while playing a video :
DRIVER_IRQL_NOT_LESS_OR_EQUAL
0x000000D1 ( 0x3D64oFAA , 0x00000002, 0x0o0o0o0o, 0xF7153C44 )
It was pointing to some cmuda.sys file.
How do I exactly find the reason of the BSOD using this information?

cmuda.sys is part of your c-media driver so you might look in Device Manager and consider reinstalling the driver.

See also Troubleshooting Windows STOP Messages

I found the latest driver for my audio device on the web.
The first thing I tried was to play a game with the audio drver disabled to see if the problem disappears.
I had no luck so I uninstalled it and I installed the latest one.
This also solved nothing.
Maybe I should consider buying an external audio device?

The first thing I tried was to play a game with the audio drver disabled to see if the problem disappears.
I had no luck so I uninstalled it and I installed the latest one.
This also solved nothing.

So with the device disabled in Device Manager, you still got the same BSODs?

So this only happens with games?