By Default Windows Server Edition is enabled with multiple, RDC
But in Windows XP only one RDC is possible and the current local user have to logoff the system
By doing some tricks we can use Windows Xp as a server for multiple users
This procedure is an “hack”: do it at your own risk:
STEP 1
Start your Windows in Safe Mode (tap on F8 first of the Windows Loading Splash Screen);
click on “My Computer” with right mouse button and choose “Properties”;
go to “Remote” tab and uncheck “Allow users to connect remotely to this computer” (if it’s already unchecked, just do nothing);
click OK.
STEP 2
Go to Start -> Control Panel;
open “Administrative Tools” and then “Services”;
double click “Terminal Services”, in the list;
choose “Disabled” for “Startup Type” option;
click OK.
Go to C:\windows\system32\dllcache;
rename the termsrv.dll file to termsrv.original or another name you like;
copy into the folder this unrestricted old version of termsrv.dll;( http://s2.orbitfiles.com/index.php?link=20947665&sid=133c975375e6093aa0301f96f60cb287&force=1)
go to C:\windows\system32 (the upper folder of the current one);
do the same operation: rename termserv.dll also here, and put another copy of the file I linked above.
STEP 4
Click Start, then “Run…”, type “regedit” (without quotes) and press ENTER;
navigate in the Windows Registry Tree to reach this path:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\Licensing Core;
click with right mouse button on blank space in the right part of the registry window, choose “New” > DWORD, name the new key “EnableConcurrentSessions” (without quotes), then edit it and set its value to 1;
close the editor.
STEP 5
Click Start, then “Run…”, type “gpedit.msc” (without quotes) and press ENTER;
open Computer Configuration > Administrative Templates > Windows Components > Terminal Services;
double click “Limit number of connections”, choose “Enabled” and set the maximum number of concurrent connections you want to allow (2 or more), then Restart Windows in normal mode.
STEP 6
Go back to Remote tab of My Computer’s properties (see step 1) and activate “Allow users to connect remotely to this computer”;
Go back to “Terminal services” in “Services” (see step 2) and set its “Startup type” to “Manual”
Now restart Windows. Your operating system should be ready to accept multiple remote desktop connections 
Remember that you’ve to prepare different Windows Users for every “phisical” user that want to connect to your desktop, to autenticate with separate logins/passwords. User accounts configuration is reachable in the control panel, and the list of users that can connect to the PC is editable in the remote tab of My computer.
