help with Logon error

I am setting up a new Win2k PC to use in our lab. Several people use this machine. As a common user name for shared tasks, I use a shared account, let’s call the account USER. when I try to logon as USER, I get a LOGON MESSAGE box stating that the machine is “Unable to log you on because of an account restriction”. I can log on as some users but not as others. It seems like an Active Directory problem but I have not been able to see the problem. I can log on to other workstations just fine with the problem user accounts.