Deleted Documents Randomly Appearing

Hi, just recently when using MS Office 2007 word documents that had previously been deleted appear open when I close another document that I am working on. The documents in question are not onces that I have created or even ever opened before, they are documents that I backed up from someone else's Laptop when I was doing it for them.
Anyone notice this before and would formatting my Hard Drive resolve this or would I need to Zero Fill it / Erase it using something like Nuke and Boot?

Thanks