I have the same problem. However, I have a spare hard drive with another copy of vista-32. With that harddrive, I am not getting the lockup. So I am guessing the problem is with a driver, not the hardware or overheating.

OS: Vista 32
MB: Asus M2N32 SLI
CPU: AMD 5600
RAM: 2 GB
Video : ATI 1650
Using integrated sound.

I'm guessing it has something to do with the chipset drivers from nvidia. I'll verify if they are the same drivers between the two installs.