My modem works fine with SP1. After installing SP2 the modem disappears from the device manager. It cannot be detected by the add hardware wizard. I have tried 2 different modems, both function correctly with SP1. Restoring back to SP1 restores the functionality of the modem. The motherboard is a Microstar 6378 but no bios upgrades seem to address this problem.
Try taking the modem out physically and then reinserting it, perhaps in a new PCI slot (I'm presuming it's an internal modem as you mention the mobo). Then reboot and reinstall the latest drivers for your model.
They are standard internal dial-up modems. I'm assuming there is actually nothing wrong with them as they work under SP1. I've tried different PCI slots. You can't install the latest drivers as windows can't find the modem to install the software for! Modems are not even listed in the device manager.