Update: Computer Problems
After reinstalling the OS because I thought one of the drivers may be causing the reboot problem and taking apart the computer, I individually tested each stick of RAM (4 x 1GB) which took a few hours. It turns out the last stick of RAM was bad, so I’ve removed it and all it working. I guess I don’t really need 4GB of RAM anyways.
I’m in the process of installing Windows Vista Ultimate (hope I like it and don’t revert back to XP).