Only games?
No error messages, warnings, pop-up windows?
Have you tried reinstalling the video drivers or any other drivers?
Try running the Windows built in troubleshooters. May take care of some problem for you.
Check that all internal cards, connections, chips etc. are fully and firmly in place.
Use Event Viewer to look for error codes or warnings just before or at the time of the freezes.
Reference:
http://www.tomshardware.com/faq/id-3128616/windows-event-viewer.html