If you ever get this in Windows 7
Another user is currently logged to this computer. If you continue, this has to disconnect from this computer. Do you wish to continue ?
That’s because by default Windows 7 doesn’t allow concurrent user access through RDP. When you are trying to log in to a windows 7 machine that already has a user signed in, you will not be able to log in and will get the warning message shown as above.
But there are ways getting around this. All you need is to replace the termsrv.dll file in the %SystemRoot%\System32\termsrv.dll
You can download the termsrv.dll here (see update blew)
Run the install batch file. and If you are not running RC, change this line in the batch file to match your version number.
However, the batch file didn’t install it correctly for me, so I had to manually replace the termsrv.dll file in the directory.
If you are running 64 bit OS make sure to replace the 64 bit one.
Once this is done, you should be able to see multiple users in the Task Manager > Users Tab.
[UPDATE Nov 16 2011]
Due to broken download link, and also there is now a better way to make this tweak easier. Thanks to this GUI patcher, you can download the tweaker here.