How to fix my computer?

My computer has been acting really strange lately. But ever since I got it there seemed to be something wrong with it but lately its been really bad. My computer shuts down like every ten secs randomly and if im playing a game on it it randomly turns off the game or freezes all the time. I dont have an internet connection on that computer either so I couldnt have gotten a virus from that... And a few months ago I downloaded kaspersky securty or something and its an anti virus system and it did find like 66 viruses and fixed them but whenever I do scans it says there are no more viruses. And yet my computer stinks. And I dont know what to do. Is there anything I could do to fix my computer ? Because now I cant even change my background or anything like that, it wont let me open some word documents, and it freezes/shuts off out of nowhere.

go to spysweeper.com and download the spysweeper and then run it and it get ride of cookies and viruses

Sounds like you need hardware update. New hard drive with clean install of Windows XP/Vista should fix it. I would recommend Windows XP professional (I have found this to be most stable). About the game freezing. Does it freeze at the same place each time ? this is usually video card problems.