If all you want to do is make sure that deleted files on the drive are unreadable, and don't mind doing it once or twice per day, you can run the following command from a batch file, or scheduled task:
cipher /w:c:
http://support.microsoft.com/kb/814599
This command will overwrite the free areas of the drive with random data in several passes. You can also run it against a folder.
If you want to be able to securely delete individual files something like BCWipe from Jetico will do the trick. You used to be able to get it free for home use, but I don't know if that is still the case.