http://www.heidi.ie/node/14 (That links directly to the d/l for Eraser)
When 'deleting' files under Windows, the OS essentially just modifies the directory entry that refers to the data's location, the actual data itself remains intact. For the data to be 'securely' removed from the system, it has to be actively overwritten. There are security standards set by organizations like the NSA (the same people who brought us ECHELON) that basically require the data to be overwritten with 0's or 1's for a set amount of times in order for the data to become irretrievable.
Deleting and Formatting disks does NOT remove the data from a drive, and anyone with some skills, proper software, and a bit of time can retrieve the data.
Hope this helps...