Depending on long its been it may be time for a format and install of the operating system. I used to do this every three months but for an average user once a year should do. Over time the registry gets messed up slowing things down and causing freezes and reinstalling gets you a whole new registry. Make good backup of important files and i would recommend formatting and a clean install. I am also including a link on cleaning from HP (the current maker of Compaq).

http://h10025.www1.hp.com/ewfrf/wc/docum...