These will not be the responsibility of Gateway or Microsoft so don't call either. Instead look for the drivers for motherboard, video, audio, modem and what you have before the upgrade and collect these so you have them ready.

If you find there is no driver for a modem then you have to decide what to do about that. If you don't use the modem, then remove the modem. If you need a modem then the cost to upgrade to XP may include a new modem.

Hope this helps,