I would like to allow concurrent users on an XP install, but I was wondering if there was a way to do it with out swapping dll's and booting into safe mode.
(Registry changes are fine)
No, that's not possible. The restriction is hard coded into the terminal server dll's provided with Windows XP. The only way to get around that is to replace them with a previous version (one of the early beta versions of Windows XP didn't have that restriction).
It's not possible to do it without patching or replacing DLLs. But some people have made "patches" and automated the procedure.
One example is the "TerminalserverNoRestrPatch", available at some places on the Net, including here.
Beginning of their info.txt file: