format won't do jack.
You need to format it and then use a hard drive scrubbing program. You can get some that boot from floppy disks. Run it several times (or as recommended by the author of hte program).
Then reinstall windows on the clean hard drive. If you rely on formatting alone, you could be in for surprises. And if you rely on just deleting files, don't count on any type of security.
Sorry, that is just not true.