the RNDIS drivers should only be needed for Win2000, not for WinXP.

after you've setup usbnetworking on the N900, connected it via USB to a WinXP host and selected "PC Suite Mode", what happens?
the "PC Suite Mode" does the same the "maemo-statusbar-applet" did for the N800, to enable USB networking.

i haven't used this under windows, but with Linux it's really easy. i'll try to find the time to write up a how-to this weekend.

btw, when the connection is established, you have to change the default route on the N900 so it actually uses this new interface.