Windows Server 2008 Remote Desktop Servers require Network Level Authentication. The RDP Client available for XP is not set to use Network Level Authentication by default. The result is that XP machines may not be able to connect with the message “The remote computer requires Network Level Authentication, which your computer does not support.” Or you might have to click connect one more time after clicking Ok to the error message.
To correct this issue configure Network Level Authentication in the registry of the client machine.
1. Click Start, click Run, type regedit, and then press ENTER.
2. In the left navigation pane, go to the following registry subkey: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa
3. In the details pane, right-click Security Packages, and then click Modify.
4. In the Value data box, add tspkg to the list of packages.
5. In the left navigation pane, go to the following registry subkey: HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlSecurityProviders
6. In the details pane, right-click SecurityProviders, and then click Modify.
7. In the Value data box, add credssp.dll to the list of Security Providers (be sure to include a coma between providers).
8. Exit Registry Editor.
9. Restart the computer.