Here is a simple regedit that will allow you to remove the annoying warning message remote desktop client version 6 displays.

1. Fire up regedit

2. Browse to the following hive key:

HKEY_CURRENT_USER\Software\Microsoft\Terminal Server Client

3. create a DWORD key named AuthenticationLevelOverride and set the value to 0

That is all you need to do.

NOTE: If you are a Vista 64 Bit user you must still create a  32 bit key.