There are still a lot of Windows XP machines floating around the business world; With Windows 7 fixing the entire Vista mess, adoption rates for Windows 7 are much better. There are plenty of die-hard users who love their Windows XP machines and won’t give it up for the world but need to connect remotely to Windows 2008 servers using Remote Desktop (RDP), which (by default) requires Network Level Authentication (NLA).
When connecting from Windows XP to Windows 7 or Windows 2008 server, you may receive the following error message:
The remote computer required Network Level Authentication, which your computer does not support. For assistance, contact your system administrator or technical support.
Windows XP SP1
Windows XP SP2
Windows XP SP3