Infra ports seem to be disabled as default in BIOS so you'll have to go into your BIOS and set an IRQ/ + I/O address for the port just like for any other port such as parallel etc. How you do this is unfortunately specific to your particular BIOS.
Don't know how much you know about BIOS so: 1)for most modern motherboards you hit <F2> on bootup to enter BIOS 2)You need to look for an option for "I/O configuration" or "COM PORTS" which should allow you to set either the infra red port to AUTO or pick a particular IRQ setting for it.Once this is done the port is enabled on BIOS 3)When you restart WIN 98 it should automatically detect new hardware i.e the IR port and try to find the most appropriate driver for it. Hopefully if you can't get the original IR port driver the generic windows driver should work for it. If not check your motherboards website for a driver update for the IR port