Windows XP search assistant* has a database of file types it uses for filtering and to determine which files are worth searching into. It also has a small misfeature where it updates this database every time you search. It doesn't send anything to Microsoft about what you're searching for, it's just an overly persistent auto-update mechanism.
The usual suspects freaked out about this, of course.
* The little dog that makes intermittent scratching noises when you accidentally leave explorer open, slowly driving you mad
anyone has a source for this? Was this built into windows? (It's not in mine, or it's not active)