First scan PC for viruses, malware, etc.
Then I would check this thread out (similar problem, solution was to update BIOS):http://www.bleepingcomputer.com/forums/t/101600/vista-constantly-freezing/
Then I would look at the Device Manager for conflicts:
* type "sysdm.cpl" (quotes optional)
* select "Hardware" tab
* click "Device Manager" button
* look for red exclamation points and yellow triangle warning signs, if you see any they might
be causing the problem. check for driver updates on those items, or even just disable them.
Apart from that, I would try to determine if there is some software conflict. Have a look at Vista selective startup:http://support.microsoft.com/kb/929135
This might help narrow down where the source of the problem is.
Hope it helps a little.