Have you ever notice that your OS and other applications seems to perform sluggish or slow, in spite you done all the necessary tweaking?
Before you think of reformatting your hard drive, try to uninstall your anti-virus and check if system performance has improved. Many problems I have encountered in the past in as far as system slow down is concern can be attributed to a problematic anti virus application.
Re-install the same anti-virus or a different one, if problem still exist, its time to reformat your hard drive to restore your PC's performance.
Have a nice day!