Wiping hard drive?

Is there any way to completely wipe off any information on your hard drive? I need to do this as it will solve problems on my spare computer, but I donot know how.

If you're using it to run an O/S then you're in for a long ride, but check out

http://www.microsoft.com/australia/smallbusiness/issues/technology/protect/harddrive.mspx

hope this helps.