You had another problem too: Windows XP x64 Edition was built on NT 5.2, the same branch as Windows Server 2003. A few software (especially antivirus software)

You had another problem too: Windows XP x64 Edition was built on NT 5.2, the same branch as Windows Server 2003. A few software (especially antivirus software) did a simple version check, Windows reports it's version 5.2, and the assumption was made that it must have been Windows Server 2003. Refuse to run because you have to pay more for a server edition.