Most NICs (Network Interface Cards) have a PCI connection, so provided there's an available PCI slot in your PC, you should be OK.
Here's an example of a wired network card you can buy:
http://www.netgear.co.uk/wired_networkcard_fa311.phpHere's an Amazon page for a wireless network card, again with a PCI slot:
http://www.amazon.co.uk/Netgear-WG311T-Mbp...r/dp/B00022V29OAll it requires is a free PCI slot on your computer (but check first to make sure you don't have an on-board NIC). To tell whether there's one likely to be free, the only sure way is to disconnect it and open it up. But if, when you look at the back of the PC, there are any slots in the case that are covered by a piece of metal bolted in, then more likely than not there's a PCI slot behind it.
Always make sure you earth yourself before handling any computer components, and also make sure the computer is totally disconnected from the mains before opening it up. If in doubt, get your computer store to do it for you; it's only a five-minute job.
Whatever card you go for, there will be instructions included. Always read those before starting - they're usually fariy easy to understand and complete. It will also tell you whether you need to install any drivers (probably not on XP).