LG 's (and Google's) drivers are installed.
P.S.: Please don't ask me to change OS on my PC because I can't.
Sent from my D620R [Stock 4.4.2]
Sorry, don't have any XP machine left.
But i would bet on the USB drivers.
Nothing else can trick the device manager.
I would try to uninstall all android related USB drivers and only install the original LG ones.
This is to avoid the wrong drive to catch the device first.
Fastboot works flawless on my side, on two pc's.
One with Win 8, one with Win 8.1 and Ubuntu Linux (on both OS)