What is the solution to fixing my vista computer freezes?

I been trying since Tuesday and I still haven't fixed it. I performed several disk checks to the point they are simple 5 minutes compared to the first time(3 hours). I defraged my disk space and I use SystemCare Free to fix registry errors and spyware scans(use spyware terminator as well). I don't know what else to do!! I had this happen two months ago, and wrote down my steps in fixing that. Nothing worked this time. Even though I don't get sidebar.Exe errors anymore, my computer freezes after 5 minutes after being turned on. What should I do? Still have 75% of unused RAM and DDR2 memory. Its a vista home premium.