there are a number of commercial programs that comply with DOD standards you can use (symantec is a popular program) or you might try some of the freeware programs (they come with no warranty however) here's one:
http://lifehacker.com/394833/wipedisk-securely-and-completely-erases-disks

 

Thanks Roger, I appreciate it. Excuse my ignorance here, but this will take care of all of my saved documents, outlook express, etc?

 

this one is very good (and free)
http://www.heidi.ie/node/6

 

Darik's Boot And Nuke

http://www.dban.org/