Need help troubleshooting

I just put together my third build, and I'm having the strangest problem. After a few hours, aspects of Windows randomly freeze.

The keyboard still responds to Ctrl/Alt/Del and the Windows start button, but that's it. No other buttons respond. And the mouse cursor still moves as directed, but I can't click anything. This sounds like a software problem on the surface. The problem is 1) I've used this same copy of WinXP Pro to reformat numerous times on my last build, and it never did this. 2) Before freezing, the computer slows down first, like it's in a really intensive process.

I've narrowed it down to either the mobo drivers, bad ram, or a defective part. I'm not sure where to go from here though. Have any of you had this problem? And is there a better place to get new drivers than here?

Thanks

(components in my sig)

i would start with taking out all the ram but one stick if that helps put in two matched set and your running two deferent speeds of ram that may do it
Rich