It’s no secret (or at least, it shouldn’t be) that when you delete files or folders in Windows, they’re not actually erased. The space they took up is simply marked as “available for use,” which allows the files to be recoverable (with the right software) until they’re overwritten with new data.
There is a utility built-into Windows (even XP Pro and Vista) that will overwrite all the free space on a hard drive, insuring any files you’ve deleted stay dead. Launch a command prompt and type cipher /w:X where X is the letter of the drive or partition you want to wipe. Be patient the process can take a long time if you have a lot of free space.