I successfully downloaded and installed Remote Desktop Client 6.0 for Windows XP today. I found it on this page.

http://support.microsoft.com/kb/925876

The page says it supports Network Level Authentication. Unfortunately, when I try to connect to a Windows Vista machine with NLA enabled, I get a dialog stating, "The remote computer requires Network Level Authentication, which your computer does not support. For assistance, contact your system administrator or technical support."

Is this supposed to work, or does it only support Vista clients? If the latter is the answer, then why would they even mention NLA on the RDP 6.0 client for XP?