If you go into the event viewer (right click on my computer, choose manage, then expand system tools and expand event viewer. Choose the application log from the list). Do you see an event, most likely a warning numbered 1517 or 1524 with a source of userenv? If so there is a file on MS's site... actually a service that may help. Let me know if you have that error and I'll hunt down the link to the file.
-Ry