we think twas an act of sabotage by an employee who is "no longer with us"
you know, you can look into that computer's Event Viewer in the Security tab, and hopefully see when the password was changed.
Start -> Settings -> Control Panel -> Administrative Tools -> Event Viewer -> Security
Ha! I did that when I left my last job. The best part is that they never figured out it was me - they thought the tech guy did it. ;) So evil.