Hi there Dave,
Welcome to
CompuForums! I hope that you post often here, even after this has been sorted out for you.
I am assuming that you would like to remove all traces on the drive? Well, it is impossible to completely destroy the data without shredding the physical hard drive, but you can run some software that will prevent casual snooping around the drive, and will make the job of professional data recovery people a lot harder, if not impossible.
You should try DBAN - it's free, and can be loaded onto a blank CD-R, a floppy, or a flash drive.
Visit it here for more information:
http://dban.sourceforge.net/
The interface isn't very pretty, but it does the job.