username and password greyed out

Hi all.Can anyone please tell me why sometimes an xp user belonging to a domain get that when they try to log on,they just get two greyed out textfields where you can’t enter anything.the username field and the password field is greyed out.you can still select between local logon and domain but that doesn’t help.the only way to solve the problem is to reboot!!!!i thought that it might be a bug in the new service pack releases but i want to be more sure than that.Thanx for your support!!!!