My computer is UNBELIEVABLY slow?

I've had my laptop since August 2009, and obviously it wouldn't be as fast as when it was first taken out of the box. However, for about the past week and a half, my computer has become so slow it usually takes about half an hour to set up Internet, and it took me the past hour just to get onto YA to post this question. I counted how long it took before it registered when I clicked something, and I got over a minute, and that's BEFORE it starts setting up a new page - usually about two minutes later. I don't think it has any viruses, I've always taken good care of it, and I'm the only person who uses it. Any ideas what's wrong? Thanks so much :)