At first you may have to follow similar footsteps by looking over the ISP's own FFAQs. Adding a router in at first was a pain here for sure!

That was solved however with a fast trip through the ISP's support pages.
I can't say this method would work well for you however depending on if you plugging into the router by usb or by a nic card installed in a pci slot to be hardwired out to the router. This would be a quick set of steps to take.
1) Review support for router as well as installation of software/drivers on XP
2) Review the Device Manager>network adapter sub section to if the adapter item is working properly with a right click for the properties screen and not seeing any yellow or red marked item(s).
Now depending on your ISP if you have an assigned user name/password you would then set up a new account in Outlook Express using that information to get mail going while if nothing is yellow marked in the network adapter sub.
3) Look at the "Network Connections" in the CP being XP there. It wouldn't be any good showing screens from the XP Mode for sure since that uses NAT virtual networking.
Your interest would be looking at the Local Area Connection Status to see if XP is all set as far as the router is concerned. Most likely XP is already synced with the router but may still need to get past that.
Have you connected with the same connection with XP previously with the router inline? If you start recycling power on both router and modem and synce the modem with the ISP while everything is shutdown you could lose the connection for 7.
The methods can be different at times between the two versions. Here while the router was inline it was still hardwired by the RJ-45 cables for both pc and modem.