If you experience the error "The remote computer requires Network Level Authentication, which your computer does not support" when connecting from Windows XP...
You will need to perform the following steps:
- Apply the latest Windows Updates (incl. XP Service Pack 3)
- Ensure that the Microsoft Remote Desktop Client is at least version 7.0 [Download Here]
- Enable Network Level Authentication (NLA) [MS Fix it for me]
Please see the following Microsoft KB article for more information: