freezing

ok, so for the past week or so my computer has been freezin non-stop. when i hit the alt+control+delete thing there's no errors (usually) so i just hit cancel and everything works fine for the next 20 seconds or so -- its like it refreshes everything or something. then i have to keep doing the alt+control+delete // cancel thing the whole time i'm on the computer until it finally *really* freezes and i have to restart it. which happens about every 20 minutes.

what the hell is this? anyone know any ideas on how to make it stop?