Do your Windows Vista machines connect to Terminal Server 2008 but your Windows XP machines won't?
First, Windows XP needs the RDC version 6.1 to connect to Terminal Server 2008, so you need to have Service Pack 3 installed.
Second, if Terminal Server 2008 requires NLA (Network Layer Authentication) then you need “CredSSP” to be turned on on your machine.
You need to modify the registry to turn on CredSSP. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully.
Click Start, click Run, type regedit, and then press ENTER.
1. In the navigation pane, locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
2. In the details pane, right-click Security Packages, and then click Modify.
3. In the Value data box, type tspkg. Leave any data that is specific to other SSPs, and then click OK.
4. In the navigation pane, locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders
5. In the details pane, right-click SecurityProviders, and then click Modify.
6. In the Value data box, type credssp.dll. Leave any data that is specific to other SSPs, and then click OK.
7. Exit Registry Editor.
8. Restart the computer.
This information was originally found on http://support.microsoft.com/kb/951608
