New computer going really slow?

I got a laptop at Christmas, and it was fine until about February, but then it started going painfully slow. I am using genuine windows vista home premium. I have 2gb of memory (ram) and a dual core processor, my 160gb hard drive still has 80gb left. It really is slow, it can't even play videos without lagging pathetically. I can't even watch youtube, and god forbid I go on a game. *sigh*