Windows 2000 AD acting up

Recently, some users started complaining about logging on to the domain. The error is "the system could not log you in. make sure your username and domain are correct and then try again" at first, i thought it was the normal password issue but after resetting the password, the problem persisted. I also suspected ACL. I purged all disabled users from the AD, yet the problem persisted. Now, with the domain admin, i still cannot logon to the domain while i can do remote desktop (with the same user: admin and password) and gain access to the domain. So strange.
Can anybody offer any suggestion please???