Sdelete is a small useful utility by Mark Russinovich that will perform a secure delete within Windows. In addition to deleting files and folders it has an option to cleanse free space on the disk. This came in very handy when I needed to clean up a PC but couldn’t rebuild it.
You can download the utility and find lots of good information about how this works at http://technet.microsoft.com/en-us/sysinternals/bb897443.aspx