passwords suddenly disappeared and are no longer needed to log on. WHY???

All of a sudden the passwords that were associated with the two administrator accounts on my computor (my girlfriends and mine) are no longer showing as registered under the user accounts control panel, and are not needed to log on. What could have caused this and what should I do? i have already set up new passwords for both accounts, but my concern is what could possibly have caused this? Any help or suggestions appreciated. I am on an HP Pavillion dv6-1134.ca entertainment notebook with windows vista home premium service pack 2. Thanks.

This could have been caused by a number of reasons. Firstly, and the most probable, a virus could have edited the SAM file and removed your password. Make sure that your anti virus is up to date and you have the latest windows updates and run a full scan. Secondly, could anyone have accessed your PC? Because if this is the case, then they could have booted from a CD and used a SAM file or NT Password editor.