I'd be willing to bet money your HDD is on its last legs. Back up anything and everything you don't want to lose, and take it somewhere to be tested.
My macbook is suddenly VERY slow. It can take 10 minutes to open finder or anything else for that matter. I have over 300 GB of space on my internal hard drive so it's be a space issues. The problem started during an import of music from my external hard drive into iTunes. I ended up having to force shut down because it wouldn't respond to anything else. Why is my computer so slow now!?!