domain problem

In my domain PCs are showing following error at logon wondow

:

The system could not log you on.make sure username ans domain are correct,then type your password again.Letters in password must be typed using the correct case.Make sure caps lock is not accidentally on.

Plz help me guys on this problem.

just for a trick they remove the network chord and login with the same username and password and they login successfully.

But when they apply this they face sometime the problem in accessing the shared files and folders on server.

plz help me out\

thanks
in advance
and cheers
rak