To enable NLA in XP machines; first install XP SP3, then edit the registry settings on the XP client machine to allow NLA
• Configure Network Level Authentication
1. Click Start, click Run, type regedit, and then press ENTER.
2.
In the navigation pane, locate and then click the following registry
subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
3. In the details pane, right-click Security Packages, and then click Modify.
4. In the Value data box, type tspkg. Leave any data that is specific to other SSPs, and then click OK.
5.
In the navigation pane, locate and then click the following registry
subkey:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders
6. In the details pane, right-click SecurityProviders, and then click Modify.
7. In the Value data box, type credssp.dll. Leave any data that is specific to other SSPs, and then click OK.
8. Exit Registry Editor.
9. Restart the computer.
Now
when you run remote desktop you will notice that Network Level
Authentication is supported. To check this, right-click the top left
hand corner of a remote desktop session and choose, Help > About
Here is the Link to Microsoft KB
http://support.microsoft.com/kb/951608/
Article ID: 67, Created On: 11/12/2011, Modified: 11/12/2011