QUOTE (Ausdoerrt @ Jan 02 2010, 01:05 PM) Defragmenting the HDD once in a while is the key. Allows the system access files faster. Also, cleaning up temp files and the registry. Still, with, for example, XP, a fresh format-and-install is recommended every 1-1,5 years for best performance.
^this, when one deletes stuff from a computer whether it be from the recycle bin or directly, that data can still be restored if you know what your doing (without system restore). As a result, the only real ways to permanently delete that data is either via formatting or defragging the hard drive. These are the two key things. Defragging can take a while and be a pain to find a real good program since Windows defrag program has always sucked, but if you are able to find one, it will be worth your time, especially if you delete a lot of stuff from time to time.
I usually try to defrag once a month in order to keep things running smoothly since I'm running a laptop. This usually allows the laptop to run 4-6 years without problems. Otherwise, be prepared to notice a slowdown in the computer after maybe 2 or 3.