can not log in windows xp machines in windows 2003 domain

have a four windows 2003 active directory parrelal servers installed.Suddenly windowsxp users in windows domain could not access to the networked systems.In the error massage it says “THE SYSTEM COULD NOT LOG ON YOU.MAKE SURE YOUR USERNAME AND DOMAIN ARE CORRECT,THEN TYPE YOUR PASSWORD AGAIN.LETTERS IN PASSWORDS MUST BE TYPED USING THE CORRECT CASE”
If u all know any solution PLS reply….!!!