WinnApp2.ini has it right. Wipe Free Space is a security feature and can take hours if not days to complete. It writes over "deleted" areas on your hard drive to erase what was once there, since with Windows pressing delete doesn't really delete anything. It just marks the space as available, but the data is still there, just incaccessible to people without skill or special programs to retrieve the information.
I used Wipe Free Space once and that was the last time I ever used it.