Andrew it is likely you may need to reinstall the drivers and/or software that came with the router. If this is a router provided by your ISP or your bandwidth provider it is also possible that you will have to contact them in order to setup the network again.
Unfortunately however I can’t provide much more details, as I don’t know what type of router you are using. In addition reinstalling Windows XP should generally not affect the router, it's also just possible that your network / USB is not setup in Windows XP.