What is happening to my computer?

It freezes a lot. It's a late model Dell deskop with a duo core and a fair amount of memory. 63 processes typically run. The commit charge usually says something like 500M/2500M, but the CPU usage rockets to 100% and locks up. Even when I have nothing open. I have cleaned the desktop, the registry and defragged. I run up to date Norton. How do I stop this from locking up 2 or 3 times per day?