Problems with "programs not responding"

I have had an annoying issue for a wee while now. My PC kept freezing while on the internet with the usual Firefox not responding message, sometimes it would unfreeze after a minute or so but would continue to do this on a regular basis. Sometimes I would try to open task manager and close down Firefox, not very successful so I would hold in the reset button and reboot the PC. I thought that it only related to Firefox but then I found it doing the same thing when just browsing through folders on my pc or when i had programs open.
Tonight it has happened twice. I thought I would open windows event viewer to see if I could see what was going on but i am unable to open event viewer, in fact I cannot open anything that is in the administrative tools folder.
Any ideas would be greatly appreciated

Thanks Chris