Windows 7 freezing, even in safe mode.

After exhaustively trying to figure out how to install Windows 7 x64 on this PC, I put the hard drive into another rig, installed Windows 7, ran sysprep, and put the hard drive back into this computer. Everything seemed fine until I started trying to install drivers and updates.

Despite undoing my changes with System Restore, my computer still freezes seemingly at random, even in Safe Mode, though it seems to last longer there. Once, the computer froze while logging in, if that helps. Aside from the login freeze, it almost seems like it's related to hard drive activity.

Prior to this, I was running Windows Vista x32 without any major issues. I'd greatly appreciate some help in figuring out what's wrong here. Let me know if you need anymore information.

CPU: Q6600
MB: Asus P5N-D
RAM: 2 gig DDR2 (x2)
Video: Radeon HD 4850
Monitor: Dell 2209 LCD