the defrag key is right next to the "any key" http://windows.about.com/od/troubleshooting/ss/defrag.htm?terms=defrag
I recommend running chkdsk Check Disk, followed by defrag once a month. Also, it helps if you defrag with your computer in safe mode, because less files have the potential to be in use. To run chkdsk. Open Start -> Run -> Type: cmd -> Enter Type: chkdsk /f It will tell you that it cannot lock the current drive and it will ask if you want to run chkdsk upon startup. Type Y -> Enter Restart your computer and it will boot into check disk. To start in safe mode press the f8 key when windows is booting up and select start in safe mode when prompted.
I came across this (researching what would be an optimal % free space for a HD to have), thought it was relevant to the discussion 4 Ways to Speed Up Your Computer's Performance