You need to manually install the driver for the controller.

Here is how:
For XP

Go into the Windows Control Panel
Go into System
Go into Hardware
Click on the Add Hardware Wizard
Click on Next and let it scan just for the fun of it ( no choice)
Click on Yes ( Already Connected)
Click on install manually from a list
Click on show all devices.
Click on Have disk
Copy Manafacturers files from C:\Program Files\RealFlight
Click on next,
Realflight driver should appear.
Click on next
Click on next
Click on View/Change resources
Click on Set Manually
Make sure use automatic resources is selected
Click on OK
Click on Finish

Reboot and you should be good to go.
Copied from anouther XP-G2 thread
Todd