Start your Windows in Safe Mode.
Open "System Properties" and disable "Allow users to connect remotely to this computer".
Disable "Terminal Services".
Go to "C:\Windows\System32\" and "C:\Windows\System32\dllcache\". Replace the "termsrv.dll" file with the Win2003 version.
Go to "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\Licensing Core". Create a new DWORD key, name it "EnableConcurrentSessions" and set its value to 1.
Run "gpedit.msc". Open Computer Configuration > Administrative Templates > Windows Components > Terminal Services. Enable "Limit number of connections" and set the maximum number of concurrent connections you want to allow (2 or more).
Restart Windows in normal mode.
Open "System Properties" and enable "Allow users to connect remotely to this computer".
Enable "Terminal services" and set its startup type to "Manual".
Restart Windows. Your operating system should be ready to accept multiple remote desktop connections.