Well, they don't make 'em like this any more - but I have a pair of USR Courier externals, V-Everything, including 90, that work very well with XP Pro. I don't have to use 'em very often, (Pimary connect is Cable) but when I do, they are very solid and tenacious.
I have installed Zoom externals for clients (V.90) and they also work very well. Just don't get a Win modem. I prefer external serial connct modems because they are easy to install - usually PnP, and can be turned off without turning off the whole system. The LEDs also tell me what is going on, send and receive.