It the box re-appears it would indicate you are not typing in the correct details.
Have you ever had to enter the details before?
Your login and password may be correct, but it might need the domain as well. So it would be:
domain\yourusername in the username field.
Use this page to find out what your domain name is.
http://windows.microsoft.com/en-gb/windows-vista/find-the-domain-your-computer-is-on
Go around to IT and pester them at the door, I am sure they will help.
Not aware of any updates that would cause this. Don't let them know you asked on Edugeek, its a sin you know.