Check your Local Security Policy and make sure that User Right Assignments are correct.

You want to make sure that your users/group are added to the Access Computer From the Network" policy and that they are not in the "Deny Login Through Terminal Services" policy.

Note that these policies on the local machine can be overriden by the default Domain Secuirty Policy (if defined.)

Also, you will want to check your Services. Make sure that "Net Logon", "Remote Access Connection Manager", and the "Terminal Services" services are running.