NT Domain Box Missing

Very strange one this.. Have a few machine which are registeried to a domain.. Works for a few days then one day when you do ctrl-alt-del the logon box come up with only username and password fields; NO DOMAIN field..
Then after rebooting about 30 to 40 time suddenly you will get a logon box with username password and domain..

Can you tell me how to fix this and what is wrong.. PC’s have been working fine for years nothing has changed on the machines since August..Nothing has changed in thenetwork infastructure either..?

Tsasha