If your comp doesn't work anymore even without the nic (as per Target's suggestion), you might actually want to check your memory... maybe you unseated it or (heaven forbid!) unleashed some ESD and fried it while installing the nic.
Read the card's manual... if your IRQ settings are wrong it could be a misplaced jumper.
Try another slot, like Target said. If that doesn't work, try putting another device in there (such as a PCI video card or modem... assuming of course it is PCI) and see if it works. If other devices work, just take the nic back to the shop and get a replacement.
Otherwise, your PCI bus could be fux0rd, in which case a new motherboard would be the only solution.