Restricting 1 Session per user in Active DIrectory

You could try this: http://www.softpedia.com/get/Network-Tools/Misc-Networking-Tools/UserLock.shtml
Alternatively, you can restrict each user to logon to only a single workstation through the logon restrictions properties of the user.  But if you want users to be able to use different workstations, but only one at a time, that won't work for you.

What's the exact problem you're trying to address, there may be another/easier way.

You can Click Star menu>Programs>Administrative Tools and Click Terminal Service Configuration
Click to Server Setting and Change Restric Each User to one session.
If you have any issue with this, you can see the link bellow for more details:
http://support.microsoft.com/?kbid=302883

I want users to be able to logon to different workstations, but only one at a time. Limiting two persons from using the same user account.

I forgot some vital info.....

1). AD is a server running Server 2008 Enterprise core
2.) Clients run Vista Business and XP Pro

You may still be able to use limitlogin
 
http://technet.microsoft.com/en-us/magazine/2005.05.utilityspotlight.aspx
p.s. article doesnt validate(just a better article than previous post)

Thanks, will let u guys know the results when I'm done testing this afternoon.