Today I was examining a customer's Lenovo X60 which failed to see its internal network card in the Windows Device Manager. It recently returned from having the motherboard replaced (cola was spilled into the keyboard).
After trying to get Windows XP to discover the device for a few minutes, I rebooted, went into the BIOS (use F1, not the blue ThinkVantage button) and reset the BIOS to defaults. Once it rebooted, Windows found and automatically installed the NIC driver. Problem solved.