There were 2 ways (known to me) of extending Windows XP to offer mulitple concurrent Terminal Sessions. One with the RC1 version of XP SP2 and one that patches Winlogon and Termsrv.dll. The latter offers not only multiple sessions but also multiple sessions under the same account and sessions to the local machine.

Seems like the someone hacked Windows Vista Terminal server too by patching Termsrv.dll. Read more at source.