A memory wiping program will only wipe RAM, but that could be accomplished by turning off the power. Are you talking about wiping your hard drive?
If you want to wipe the entire HD then yes you would need to reinstall the OS and everything else. Though many programs exist that will simple wipe HD "free space" PGP has this feature that seems to work well enough.
Personally I run (and recommend) CompuSecs HD encryption suite that way I don't worry about having to have independent programs try to keep up with cleaning the snail trail of information Windows sprays out all over.