Could be any number of things, but personally i prefer to at least do some simple things first, have you checked the Error log (control panel > admin tools > event viewer) see if anything obvious appears? Also i always run an error scan and repair on the hard drive, just on the off chance.
Did you use the system restore and take it back to a time all was well?
Does the trouble occur when running windows in safe mode?
Does the problem only occur when your running Winmx?
It could be a hardware fault/conflict, i would start by removing non essestial items, printers, scanner, ex hd's etc first then see if the problem is fixed, then add back 1 at a time. You could aslo make sure latest drivers are installed.
Could also be a problem with your ram and any number of other things.
Lots of things it could be, process of elimination really, and hope you come accross it.