Well... I tried the memory. It doesn't seem to be the issue. I used each individually, but could not keep the computer running long enough to run memtest.
On another note, I believe it may have something to do with my processor. I found that my heatsink didn't go on correctly the first time I put it together. The mounting was an issue because I couldn't get all 4 sides to stay down at the same time. I eventually put enough pressure on it to get it on correctly. After this, the computer ran for a good 3 hours allowing me to install XP. However after this it restarted and since has been restarting more and more frequently.
It restarts 90% of the time within 10 minutes now.
EDIT: I've been using a program to monitor the temperatures, but it doesn't seem to really ever exceed 47C. I dunno...