Hacked and Administrator Rights Taken!

I have 4 computers at work of which only one had a remote desktop connection set up. This computer seems to have been hacked. This person took away our administrator rights on the computer and set up a new user name called "Support" and set up a password on the username. This person attempted to Log in to our computer 3 times last night, and in doing kicks us off. I have disconnected the computer from the network for right now. Other than Formatting the computer is there anything i can do. I could not even take Remote desktop away since the computer tells me that my user does not have the rights to do so!
Thanks So Much
Samer