I have Windows XP, everytime I try to convert a video file into mp4, the progress reaches to about 20% and then my computer just restarts. It doesn't do it when I convert YouTube videos, only with movies. This only happened recently, it worked fine before. I have about 6 GB of memory and I have no viruses, what could be the problem?

In order to try and help, please tell us the following:
1. What program or utility are you using for the conversion
2. What file type are you converting from
Regards.

I am using a program called iPod Converter For Free, and I am trying to convert avi to mp4. Also, I have cleaned the fan of the computer, so I don't think it's that. I used to convert lots of videos with this program and one day I left it over night converting and then it restarted.

You could try a cuple of things:
1. Try deleting the free converter and downloading it again just in case it has become corrupted or damaged.
2. Since you are on WinXP, try "rolling back" to a "restore point" before the problem occurred.
HTH.

I tried to system restor but it doesn't let me go back to a different month, and all the checkpoints don't work. I picked three different days, and the computer wasn't able to restore it. Is there anything else I can do?

Yes,
Try "deleting" the converter using "Add/Remove" programs in the "Control Panel". Then download it again and re-install. These utilituies can often become corrupted.

I've already done this with several other programs, and it always keeps happening, I might just give up video converting, what do you think?

That's up to you but you won't solve anything by giving up!
I've just downloaded the free ipod converter and tested it on one of my small avi files.
All works perfectly, so it's not a converter problem, it's something on your PC.
Are you sure you have enough disc space?
How big is the avi file you are converting?
If I were you, I would try and remember what yopu have done, what programs and updates you have installed since it last worked, then uninstall them all one by one until the converter works again.
It's most likely a clash of programs or updates.
That's why removing (uninstalling using Add/Remove programs) the converter and re-installing is the best start.
Have you actually done trhat yet?

Well, I've already deleted the converter, then downloaded and installed another one, and it does the same things. I did a disk defragmintation and it didn't do nothing. I have about 5GB of memory so that can't be the problem.
I don't know what else to do, it just happened from one day to the next.

OK,
1.
If you've tried converting this file with several different converters, then maybe the avi file itself is corrupted or damaged near the end.
Try converting a file you have converted before which worked ok.
2.
If you have 5GB of disc space, that's not much free space. XP needs more than that itself to work.
To convert avi files you need lots of space, at least the same size as the file you are converting, for temporary files etc. and for the converted output.
So try deleting some stuff to give yourself at least 10GB of space.
3.
A useful guide for disc space is to see how much space your system takes up, say it's 20GB, then you should have at least 50% (another 10GB)for it to work efficiently.