I dont know what happened but suddenly, our computer is acting strange. I can't open the 'My Computer'. But I can still see the files in there when I browse it through online websites..
Another problem is some software cant open. And one software, I opened it and it hangs and so i need to restart our PC.
Another problem again, when downloading files online, the browser freezes.. It also happens when uploading something from the PC to the browser..
I tried scanning if there are problems in the PC, but it says that there are no viruses.

When I opened our PC now, it seems that my brother reformatted the PC. All the softwares are deleted. But the problem is still happening.

Help?

If the hard drive has been reformatted, then evidence of the problem has likely been destroyed. That's just one reason a format and reinstall is ALWAYS a last resort option. You say you are downloading and uploading files - please understand if these are songs from P2P sites, it is very likely your system is infected and will continue to get infected over and over again. P2P sites that cater to filesharing of copyrighted materials (songs, videos, and other published documents) without collecting required royalties are a prime source of malware.

I don't know what you mean when you say you can't open My Computer, but you can see the files when browsing online websites.

Have you scanned for malware? What do you use for security?

I recommend purging the hard drives of clutter with Windows (XP, Vista or Windows 7) 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.

Repeat for all computers in the "Trusted Zone" of your network (your side of the Internet "gateway" - typically the cable or DSL modem).

Windows Disk Cleanup Note: One distinct advantage Windows Disk Cleanup has over all other cleaners is that it also has the option to delete all but the most recent Restore Point. This can be significant as malicious code can hide there too. But I caution this option, found under the More Options tab, should be used only after careful consideration. If experiencing a problem that first appeared within the last couple days, do not use this option until sure System Restore can not be used to fix it. If the computer is running fine otherwise, and you are doing your weekly supplemental scans and want to root out as many potential malware hiding places as possible, I recommend setting a new Restore Point, and then use this Disk Cleanup option to clear out the old points.

@Digeratie:
This problem is already happened before reformatting it (Im still not sure if my brother reformatted it. I dont really speak with him about stuffs like this really).
What do you mean p2p? Any sites that is interacting with the PC's memory or files (like uploading, saving a file, downloading etc.), will freeze the browser. I was thinking of re-installing firefox but it involves downloading online so it'll probably freeze my browser..

Yes. I really can't open 'My Computer, My Documents'. When you click on the desktop icon or even at the start button..

I use Malwarebytes Anti Malware. I already scanned the PC before and after reformatting the PC. And there are no virus found.
When I opened this PC, saw this 'HiJack This' software. Maybe my brother downloaded it..

Ill try to download that CCleaner. But yeah, downloading online will freeze my browser. ;_; but Firefox has a restore feature so probably when I open my brwser again, I will see that the program is downloading..

I seriously dont understand your Windows Disk Cleanup Note..

I am planning to show you some screenshots of some problem.. But cant. Since if I host that screenshot online, browser will freeze like hell.

This problem is already happened before reformatting it

I understand that. I am saying that formatting likely destroyed evidence of the source of the problem which means it also destroyed any clues to prevent it from happening again.

P2P (peer to peer) sites allow users to easily download and upload files, typically songs. P2P sites are frequently used for "filesharing" of such songs without paying for them. If copyrighted, that's illegal, but for the purpose of this discussion, this practice is very dangerous as P2P filesharing is a primary source, and method of distribution for malware.

Don't worry about the Windows Disk Cleanup note concerning System Restore - it's too late for that. But I recommend you use Disk Cleanup to rid the system of clutter. This works in a similar way to CCleaner, but it is part of Windows so it does not require any downloading.

If HijackThis is installed on your system, I recommend you post a log according to these HijackThis Procedures.

Where can I even find the 'Disk Cleanup'?

I did the HiJackThis log thing.. And made a thread about the log.. http://www.d-a-l.com/help/spyware-ad...tml#post227887

Where can I even find the 'Disk Cleanup'?

I posted the instructions in my first post above. Click on the link for your operating system, XP.