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per Microsoft:

Here's the problem: An index of files is maintained for the hard drive, telling it where things are stored. When you install a file, especially a big one, it is scattered around the hard drive in bits and pieces. On your command to open the file, the hard drive checks the index, then gathers the pieces and reconstructs them.

When that file is deleted, the links between the index and the file disappear. That tells your system that the file is no longer needed and that hard drive space can be overwritten. But the deleted file remains on your computer. Only when it is overwritten do you begin to be safe. Even then, a specialist might be able to recover the old data.

Assuming you just deleted everything in preparation for saying goodbye to your PC, it is unlikely that the sensitive information has been overwritten. It's still sitting there, and anybody with a shareware program could find it.

...

Reformatting will keep most people out of your old files. But specialized shareware exists to reclaim files after reformatting. If you do not know who will get the computer — or you do know and you don't trust them — stronger measures are required.