Computer trouble, any help?

My computer is running on Windows XP, it has worked absolutely fine since the whole time I've had it. Until just recently none of my programs will open. (WMP, Lime wire, MSN) nothing, and when I try to open a program it will just sit for a minute or two then the whole PC will freeze and I have to restart it. The only thing I can do it open Firefox and browse the web. I tried scanning my PC for viruses with AVG but it will only get to a certain point then freeze. What could be going on?