Why is my computer slow all of a sudden?

I haven't had my computer for a year yet and now it's screwing up. On start up with about 5 minutes into it it works PERFECTLY but after that it will freeze up, then it will lag a lot. After the constant lag it then freeze up for about 5 minutes. I have a Dell Inspiron, running Windows 7. I have Webroot AntiVirus and Spy Sweeper installed and it's not detecting anything, I uninstalled all of the useless crap I had. I Defragged my harddrive. I did all of the things that I could think of that would help and still my computer is crappy. Anyone help me deciding what the problem is and/or what I can do? I feel like throwing this laptop to a wall and go Office Space on it!