Windows XP and above includes a free, secure disk wipe capability.

Background

When files are deleted, the directory entry is removed, leaving the data in-tact.  Commercial and open source software is available to recover deleted files, by finding the disk cluster chain, and restoring the original directory entry.

“Secure delete” means overwriting the cluster chains for a given file.  Likewise, a “secure wipe” overwrites all unused clusters on the drive.
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