Macbook pro freezes randomly and blue screened on me?

I have a 15" unibody Macbook pro that I've had for about 2 years and has been wonderful, never had any problems with it until recently it has started freezing with the spinning color wheel. I have mac os x 10.6.8 installed on the computer with 4gb of ram. For about a week it has been freezing randomly, and acting a little slow, and someone suggested making another account and seeing if I still have problems on the other account. I have done that and have had no problems. I went back to my old account that I was having the problems with in order to try to back it up, and after about 3gbs of back up the computer blue screened on me, I haven't experienced this yet. I have anti virus software on it and it says I don't have any viruses. Thanks for any helpful insights