We have several users that sporadically have issues with their default printers changing on them without them doing anything. We've had some that change after printing an email from someone, printers changing while the person is away for a while, even in other applications they are changed. The default printer is fine at boot but can change anytime during the day. We have tried removing all printers on the computer and the drivers and re-adding them all, running spware scans, and virus scans too. This is happening to Windows 2000 users as well as Windows XP but I would have to say a majority of them are Windows 2000 systems with SP4.
When the printer changes, it doesn't switch to the same printer every time - it bounces around to all of our printers on the network.
Please let me know if there is any more information that is needed and I'll try to get it for you.