my computer restarts after completion of booting

[ballhog]

This problem started about a week and a half ago.  I don't remember installing anything new, but that doesn't mean that I didn't...bad memory.  I was having trouble closing tabs in Internet Explorer immediately before this started happening.  I would attempt to close a tab and it would freeze the screen for a few minutes.  I didn't have a problem closing the whole window.

After rebooting, my computer gets to the desktop and immediately restarts.

My specs are:
Windows XP Service Pack 2
Internet Explorer 7
Gateway computer

I can start the computer in safe mode without it rebooting.  The FAQ section said that this means it is a software problem.  I'm decent with computers, but I have no experience troubleshooting software.

I would be grateful for any help.  Let me know if you need more info as well.

[Broni]

Do you have Windows XP CD?

[ballhog]

I'm pretty sure I don't.

[Broni]

It may be overheating (did you check your computer temps? did you clean your computer inside using compressed air), bad RAM (try to re-seat it; if you have more, then 1 RAM stick, try to start computer with one stick at a time), video card (if you have separate video card, try to re-seat it)