NY Times has an article on (many times unknown fact) how simply deleting data from your hard drive dosen't really delete it. This can be a problem, when you are getting rid of your old computer or are trying to erase any evidence :-) The article provides some good tips and free program links. Most notable is the FREE windows program Dariks Boot and Nuke.[Deleted but Not Gone NYT]
[Darik's Boot and Nuke]