Windows 98 users have problems

I have an Active Directory, and I’m facing this problem. Windows 2000 Pro clients can login to the domain with no prob. but windows 98 clients cannot. They use to login normally but suddenly with no warnnig this problem begins. They receive this message: “The domain password you supplied is incorrect or login to your domain is denied”. The users password is incorrect, even I cannot login to the domain. One of the guys suggested that I create accounts for computer in the AD, it worked for some time and then stopped again. Now, I have win98 stations that can login and others that cannot. Win2k Pro clients is not affected, they can login to the domain with no prob.