Could be almost anything 
I would start with the basics... go into safe mode and do a thorough scan disk on the whole computer. Also in safe mode do a complete defrag. Then I would reboot and scan with spybot/adaware and run your anti-virus program AFTER you update it. Then reboot again and see how it runs. If it's still acting up post back here and we will trace it down further.
I would start with the basics... go into safe mode and do a thorough scan disk on the whole computer. Also in safe mode do a complete defrag. Then I would reboot and scan with spybot/adaware and run your anti-virus program AFTER you update it. Then reboot again and see how it runs. If it's still acting up post back here and we will trace it down further.