Technology known as "volume shadow copy" (shadow copy of the object), and Vista is called "previous versions".
It keeps backups older versions of files while preserving the new version - the same way as Word creates "bak" file (besides this technology truly useful, as it provides you with the functionality and interface for reliable recovery of lost data).

With shadow copy, you can erase a few paragraphs in Word, save paper, and then restore lost paragraphs, if you need to.

But Microsoft says that this feature could also help in the event of accidental deletion of files:
"If you accidentally delete a file or folder, the function of previous versions can help you restore them, to restore a backup (made through Windows Bakcup), or make a copy."

This functionality is automatically included in Vista, but can be disabled via the System Protection tab in the System Properties applet.