Unless you have ACPI turned on your modem and video card should not be on the same IRQ. My guess is the modem has been put in the PCI slot nearest to the AGP slot (video card). If the agp slot is in use the first pci slot should not be used. I would remove the modem in device manager, shut the computer down, and then move the modem to another free slot. When you restart it should be redetected automatically.
Another thing you should try is removing the little flashing modem icon from the taskbar. That's usually found by opening network neighborhood, connections menu, settings, uncheck 'show icon on taskbar when connected.'