Logon credentials Supplied is incorrect xxxxx

Every time i m trying to login in my exchange server using MS-Outlook or even Outlook express errors comes that Logon credentials Supplied is incorrect, Check ur id and password.......!!

it was working perfactly but since last 1 week we r getting this problem on some of machines with xp and 2000Pro. while some machine r working perfactly. able to ping the server ip. and my server is able to machine. login into domain. able to map the machine. ad is ok but intra mails are not working.