PC Has Become A Snail

My PC has begun to run really slow, its taking forever to do the simpliest tasks. I am running XP and have run the virus scan and spybot and although they usually pick up something and remove it nothing changes with the speed. I have tried system restore as well but that seems to have been disabled by something, it goes through the motions but when the PC reboots it says was unable to restore to selected date.