Wiping the drive and reinstalling the OS is the way to go (actually, removing the drive is the way to go...) but if you can't do that, simply deleting everything, then running a utility that cleans the registry and temp files (such as ccleaner), then running something like Tolvanen's free Eraser prog would be pretty good. The Eraser prog can wipe all the free space on the hard drive, and also clean up the file table.
While deleting stuff, don't forget your emails, browser data (especially if you use it to remember passwords), etc.
There's lots of software that will do roughly the same job.
Update: I see Tolvanen's Eraser is now being maintained by Heidi, at heidi.ie.
Last edited by Bushfiva; 4th Feb 2009 at 09:59.