You need to make sure individual user accounts have terminal services access to terminal services. The administrative group by default is allowed to log onto terminal services for administrative access. However, licencing terminal services for usersneed to go through the microsoft clearing house.
To check if individuals have terminal services access. Go to administrative tools--active directory users and computers . Then click on an individual user--properties--terminal services profile--(there is a few options to be set here if not already set). I think this is mostly an individual thing by design.
Another way to allow users to log onto terminal services is to configure it through the terminal services configuration snap-in, then go to connections and set how users are allowed to log in. ie under connections--(select a connection)--then condfigure the properties.
I hope that helps.
ps If you want to use unix based machines, by all means there is third party software thatlets you utilise terminal services.