Serious problem with system (please help!)

Hi,

So here's what's happening. A couple weeks ago, my system (windows xp, athlon 5800 , asus hd3650, cheap Soyo mb) would just lock up, and was having lots of trouble booting, basically some catastrophic error somewhere. So, I replaced the motherboard. Still having problems. I replaced motherboard again (gigabyte s-series), got new RAM (kingston hyper x 8500 2gb), new hard drive (caviar black 640gb) and a new PSU. So basically everything was changed except processor and video card. So, on a clean drive, I install windows 7. Everything seems fine, then after about 3 minutes, windows locks up. the screen goes blue, then all these patterns and colors and lines start moving across the screen. About 15 seconds later, the computer reboots. This keeps happening, so I go to swap out the mobo for a new one (during this time I removed the video card to simplify the troubleshooting, and also tested other sticks of ram). SO I go and swap the mobo in, everything boots up fine, doesn't lock up after a few minutes... good news! Then i try to finish installing the utilities on the Gigabyte utility DVD that came with the mobo. It installed one of the utilities, and automatically restarted (to finalize the installation perhaps). Ok, then on reboot, it gets to the loading windows screen, and when its about to load, POOF! A black screen with some green lines running diagonally. At this point I'm going insane (mind you, this whole process has taken place over a couple weeks, most of it being in the last few days) so I restart, then another similar lock up, weird artifacts on screen, windows not initializing completely. I let Widows do a memory check, it tried to look for and fix problems, etc etc. But it's still inoperable.

So, my plea is... what might be causing this? Could it be the CPU? What do you guys think? Seen anything like this before? Please help me, I am going crazy.

Thank you!