In the case of the new computers, boot into dos, enter the command "format c:\" without quotes, and wait. This is enough for most. If you're still worried, check the maker of the hard drives, go to their web site, and see if they have a low level format tool. Maxtor has one that works on most (powermax0 but it's best to use a brand specific tool if you can. As for the machines in use by others, the person in charge is in charge of that.