A number of YouTube users replied to my video Find Your Network Settings in Windows XP asking for advice on what to do when you have no Internet after reformatting Windows. This is almost always as simple as installing the correct Ethernet or wireless driver for your computer. Your computer should have come with a disk with all of the drivers for your hardware. If you put in an ethernet or wi-fi driver after-the-fact, that should have come with a disk too.
Install the drivers from the disk, and your Internet connection should be available. if you are using wireless, you may have to configure your network name (SSID) and password.