Is the ethernet built-in? Try window's update, it finds the driver half the time...otherwise look up your motherboard's site (if self-built), or manfacturer (HP, Dell etc)'s site and find the driver. Same with if it's just a card...WinXP should have a generic driver for it, anyhow... but finding the original driver on the manufacturer's site (Netgear, Linksys etc.) never hurt anything!
Plugging the cable from router->computer should be useless considering there is no driver? But if your browsing some sites...you would already have a driver?
Sounds very odd for a fresh install...is this XP?