have you checked for malware or spyware? i see you've checked for viruses but not the others.
also, have you done a defrag of your computer HDD? fragmentation on the HDD can slow the computer down.
Although i doubt fragmentation this would be the case as it sounds like to me something software related as hardware normally doesn't do thing suddenly like that.
My last question will be, when computer posts (if you see it), what speed do you see the cpu at? it should be 2.6Ghz.