I have a G25 on XP and this is what I did to configure it.
Plug in all cables to the wheel.
THEN plug it into the pc(USB) and it'll selfcalibrate(rotating from 100% left to right turn).
Install the driver software either downloaded or from the driver cd. Follow instructions 100%.
Open the Logitech Gaming something and a picture of your G25 will pop up and show you all the buttons on it.
Go to settings and unified settings(don't remember 100% if that's what it's called). And remove the hack on the unified gas and break axis. That'll set the brake and gas pedals to seperate axis'.
Boot up LFS and go to settings. Choose your G25 and find the settings for the brake, speeder, clutch axis. There you can set up all your axis needs
Next is the gear. You'll have to set the gears up as buttons. Under the gear tab thing you'll find all the buttons and when you click ex. 1st gear you'll have to set the stick into first gear. It should recognise it.
That's how I did/do it when I plug my G25 in. Minus installation ofcourse.