Computer restarts suddenly or freezes

Hello, I'm trying to reinstall Windows on an old Asus tower that I inherited from work. I found a bad memory stick was keeping Windows XP in a boot loop, so I maxed out the system with four 1 gig memory sticks (formerly on two 512 sticks). now the trouble is that when I reinstall Windows (having formatted the drives as one of my potential solutions), it gets most of the way through the installation process, asking for my name and the key code, but it restarts while I'm putting in the information, or freezes before getting anywhere.

I thought it was some sort of time limit, so I left it on without the CD so that it would hang and ask for the disk, and so far it hasn't restarted (going for the past ten minutes). that indicates to me that the system is not overheating. The problem could lie in the hard drive I tentatively installed to get the OS working. When I tried to start the installation initially, the OS found the RAID card that was installed, but apparantly didn't realize that it was connected to two 160 GB hard drives. It told me no hard drives were installed, so I pulled the Seagate 40 GB drive from another de facto computer harvested from work and hooked it in directly to the mobo, but I can't seat the drive properly in the chassis because it's blocked by the memory, video card, and pretty much everything else in there. so it's just sitting to the side, but I've made sure it won't move around easily. (should I be looking for a way to remove the faceplate to get the drive in?)

so yeah, the computer will either restart all of a sudden, or freeze in the process of the installation (as it has now), or freeze on the XP boot screen, which it just did.

What could be ailing my computer? Thanks in advance for your considerations.