Another vote for formatting if the OS has been on the computer for over 9 months/1 year. I format my "C" drive at least once a year as Windoz loads so much crap that you cannot delete that its needed.
You do not realize how slow the 'puter has gotten until you clean it all up and reload. Until doing that, as stated above, clear out the crap out of your hard drive, get at least 25% free space on all drives and defrag the hard drive.