Computer is slow and freezes

I don't know what part of forum should I post this, sorry.

I bought my computer January of 2008, it's Dell inspiron 1521 windows vista. Some days my computer is just running fine and then just freezes like couple times a day. I tried, defrag, disk cleanup, mcafee virus scan (no virus), windows update and SuperAntiSpyware. Still freezing.

Maybe I need to delete some files that are unnecessary, but I don't know which one cause I wasn't sure and scared that my computer won't work.

Thanks.