When was the last time you cleaned inside the case of your PC?

This CNET video may help;
http://reviews.cnet.com/4520-11319_7-6240575-1.html

Do you see any Blue Screen? (Blue Screen of Death, or BSOD). See if the Event Viewer logs these events. Goto Start > Run, type in eventvwr.msc can click OK. Double click any entry for more information.

Mark