Quite a lot of hardware that worked perfectly well up to, BUT NOT INCLUDING WinXP(Home or Pro), is not actually compatible with XP, they even have a list somewhere on the MS website.
I tried three internal modems that wouldn't work, finished up with an external one that does.
If you can reach the website of your modem manufacturer see if they have a driver update for XP, sometimes that is all that is required, other times you have to change the hardware.
