When you "delete" a file, you don't necessarily remove all its content at once, you merely flag the area of disk it occupied as being available for use by the system. There could be some illegal information left after deleting a file, which you cannot access in the normal course of events.

There are several tools available for clearing unwanted information out of these areas (and for recreating files you deleted by accident); NORTON for example.

However, running a DEFRAG on the disk will usually clean off 99% of anything unwanted, unless the disk had very little information on it.