When I switched to XP I had the problem, yet again, of finding a driver for my SupraExpress 56i V Pro modem.
SonicBlue, who took Diamond over, ceased proper support and so I've used click here which is excellent. You should find the right driver for a 33.6 Diamond modem - it's easier if you have the modem's model reference number, such as SUP2560.
Even keying in the former Diamond websites brought no joy previously (website unavailable was ususal message), until I went to modem-help and found similar links to those of scotty. The second is particularly good.