Severe loss of preformance.

Every time i turn on my PC, theres a severe loss of performance when windows starts (And the CPU's fan works at 100%, making this loud noise)....what could cause this? i had the issue before, so i formatted, but the problem is still there.
One of my friends told me that it could be a boot sector virus, i doubt it...hardware issues? perhaps, but i updated all my drivers, and tried everything.

PC = Compaq 5650LA

BIOS update/CMOS battery reset? Seems like a good place to start. Following that, since it returns after a format.. Maybe your drive is dying? I can't think of anything else that would do something like that, unless your CPU somehow 'broke' halfway- but I've never heard of that happening.

http://www.passmark.com/products/diskcheckup.htm