If you cannot identify it in Device Manager, you can open the case and look at the card to find the manufacturer and model number - if it's a name brand, you can google the make and model driver.
I've install WindowsXP from the scratch and my network card has find as an unknown one (in Device manager is question mark where should be network adapter) ad cannot find an appropriate diver. I'm looking for a program which can find all hardware component and names of according drivers in a PC box. I found one such program "DriverDetective". Just using its scan to get a list of all devices in the box it's free. It is no need to open PC box.