Vista - Asus EAX1600Pro in Vista RC1

System is mobo Asus P5VD2-MX, 1 GB, Pentium D 950 LGA775 3.40 GHz.
Self-built, running XP SP2. 3 months later, I installed new Asus EAX1600Pro
512 MB graphics card, using XP driver, and have downloaded a specific Vista
driver which is annotated as suitable for Vista RC2 only. Dual-boot Vista
installation had been running well until I fitted the EAX1600Pro, but my USB
modem not recognised by Vista, so could not update driver into Vista directly.
Tried uninstalling Vista RC1, but after re-install system hangs (blank
screen) during first startup, and I cannot work out how to make the driver
available to the Vista installer, nor am I confident that it will work.
Starting in safe mode works but does not allow installation to complete, and
Repair function in installer doesn't (function, that is).
I want to complete installing Vista, but not if it requires buying new
modem. Can anyone please help ?