In windows 7+, when you enable the Remote Desktop connections, you get to choose the type of Authentication, as displayed on image below:

In Windows help, it says like this:
Select Don’t allow connections to this computer to prevent anyone from
connecting to your computer using Remote Desktop or RemoteApp.
Select Allow connections from computers running any version of Remote
Desktop to allow people using any version of Remote Desktop or
RemoteApp to connect to your computer. This is a good choice if you
don't know the version of Remote Desktop Connection that other people
are using, but it is less secure than the third option.
Select Allow connections only from computers running Remote Desktop
with Network Level Authentication to allow people with computers
running versions of Remote Desktop or RemoteApp with Network Level
Authentication to connect to your computer. This is the most secure
choice if you know that the people who will connect to your computer
are running Windows 7 on their computers. (In Windows 7, Remote
Desktop uses Network Level Authentication.)
So, if you select the third option (Allow connections only from computers running Remote Desktop with Network Level Authentication), this supposedly should allow only users connecting from Windows 7+ to connect.
Also, I am not sure if this was fixed in Windows XP SP3.
I am not sure if this is applicable to your case.