Recently, my friend built me a computer and I thought it was going to be great. I tried installing XP on it the first time, it came up with some errors about how it couldn't find some files. It worked on the other computers so I just installed it anyway and continued. I've tried this with many different XPs and it comes up with the same problem, even though it works on different computers.

Anyway, getting straight to the point, my computer likes to restart by itself occasionally when I start to play a game of Halo PC. That's not the only problem though, each time I restart my computer, a different software installation has an error when I try to start the program up. Sometimes I start up Adobe and it gives me a memory error. Sometimes I start up Windows Live Messenger and it gives me an error about not being able to talk to people.

Most of the things that are happening are pointing to a RAM problem, which is what I thought. I even ran MemTest86 and it came up with a bunch of errors so I got my RAM replaced and the same errors still come up.

I'm starting to think it's either the motherboard or power supply (which is why I'm posting it here). Everytime I try to plug in a USB cord or headphones or microphones into the front, my computer restarts.

This has been happening for a while now and it is affecting my online business. I hope you guys can help me.