Might want to run some malware scans, too. You can use one of the bootable (USB/CD/DVD) "Rescue Disks" available from several reputable security vendors (AVG, Avira, Kaspersky, Eset, BitDefender, Avast, etc...) to provide the in-depth "2nd opinion" (to help back up your locally installed antimalware). The CD/DVD disks (if not rewritable) provide a very "clean" boot from which to run the scans (helpful, since the local protectors might be compromised). The Rescue Disks are also usually free.
You could also check in on some of your basic settings and make sure they are OK -- free disk space, virtual memory settings, startup programs, Windows Updates (if some are failing to install & then trying again over and over - that'll slow things down a bit), BIOS/UEFI settings (sometimes a power surge can throw the settings off)... that sort of thing.
Of course, follow FreeBooter's tips as well - temps, clean boot, Safe Mode -- all these should help narrow down the possibilities.
Hope it goes well
