I just had this problem, it drove me crazy. I t took deleting old games and junk off my hard drive which was 70% full. run full virus scans Adaware and spybot, then deleted all internet files and cookies, then defragged it. Finnaly after all that I restarted my computer and it now works fine
Is your hard drive getting full?
Also I noticed that though all other programmes where shut down I still had 28 systems running in the background behind the scenes! Does anyone know how to shut some of these down and which are safe to shut down?