First try looking in Device Manager to see if it's there. If not run Add New Hardware. If present, but yellow flagged, Uninstall and Refresh.
Personally if that doesn't work, I'd go into the BIOS and disable some ports -- COM, Parallel Printer (if using USB printer) Game/Midi. Restart Windows a couple of times to sort out resources and have another go in Device Manager.
If it's a desktop computer and all else fails, add a PCI NIC -- very cheap and the computer won't care about having two NICs in any event. For a laptop add a USB or PCMCIA (aka PC Card) version.