LAN cards can be picky, i had a computer not recognize a wireless network adapter, but by installing the software/drivers ahead of time then installing the hardware it worked fine.
Another option, if you've had other LAN cards installed, be sure all of their drivers have been removed, as your OS may be searching for them and ignoring the hew hardware it doesn't recognize.
*depending on your comfort with hardware, try plugging another pci card into the slot, i've seen mobo's with a faulty slot, unfortunatly for me, it didn't not work, it was actually outputting too much power and took my tuner card and frontside firewire with it. The board works fine, i just can't use the single slot.. ugh
*Giving us the OS and current update level (eg Windows XP Pro, SP2) would help tremendously!