|Use Task Manager or install Process Explorer and see what processes is running in the background. Then type msconfig into run box and start it, go to the Start Tab and uncheck all except Microsoft and your antivirus program, Apply and restart the system. Recheck how it is doing.|
Note that some antivirus software actually wastes valuable system resources, if you are using Norton or McAfee, uninstall and use Avast Free version or MS Security Essentials.
You have to be a little bit crazy to keep you from going insane.