I have the CDs that have the drivers for all the devices that are connected to my PC. So, I check the device manager for the one that shows the question marks icon. If it is under the category of Display, then I would assume that it is display driver and I would install it. If it is under the category of Network Adapters, then I could install the driver for the LAN card. Like that...

I googled for "how to identify missing drivers in windows xp" and found many results. Maybe, you'll get some idea from them.