Even formatting a drive will not necessarily prevent data from being restored; GetDataBack software can restore files after a drive has been formatted.

If you're concerned that someone might go digging around and intentionally trying to restore files you've deleted, you want to use a secure erase tool. These delete the files, delete the FAT, and write random data patterns multiple times over the sectors where the files were stored.

"Eraser" (the 2nd one) can either wipe the entire drive or selectively wipe files without destroying the OS files, so if you don't want to mess with reloading and know for sure where all your files are, that might be a quicker solution.

Here are two that are either free or very inexpensive ($20)

http://dban.sourceforge.net/

http://www.heidi.ie/eraser/