We've had some NT boxes at work where an "upgrade" of IE 4 to IE 5 caused IE to ask for the "Windows" password to unlock previously cached form fill-ins, etc. The catch is that the user's NT domain password couldn't unlock it. Neither could the local admin, or the domain admin. The users in question can't remember ever defining a password, so we had no clue what it was looking for.

In the end, we got around it by deleting everything out of the IE 5 directories that didn't look like it came with IE 5. Unfortunately, I don't think that'll be of much help in this case.