Inadequate memory, too many antivirus or antispyware programs installed, defective or failing CD-ROM or DVD, corrupted software, spyware or virus... Just about any hardware problem, dirty or failing CPU fan, lint and pet hair in cooling channels... or any software that is out contacting the home site.
Disconnect the computer from the internet to see if that changes anything.
If using Windows XP, go to Start ->Control Panel -> Administrative Services-> Computer Management -> Event Viewer to see if any red or yellow flags are posted which are relevant tot he times you had problems. Click on those items, and they will tell you what error was a problem at the time.