One of the main reasons for this will be an IRQ conflict. If you have another PCI slot spare, put your network card into that, then try it.
Windows 98, unlike x\p, does not give you virtual IRQ's. If the above doesn't work, you could enter the bios and disable one of your two comm ports. if you use a dial up modem and find it does not work IF you disable one comm port, then you must delete the modem from device manager and reboot. You can then reinstall the drivers for the modem and be up and running.