I had that problem with my Sound card and modem. One day, my modem's driver just disappeared from the device manager and couldn't connect to the net. I found out that xp thinks that my sound card has a modem also for some reason so it just thought to use my sound card's driver and deleted my modem driver. I had to reinstall them two times and after that a couple of days later it did it to me again. The first try to reinstall the drivers worked.