Networking / Internet Issues on XP

I am trying to resolve this today:
Got a home network on a router. 3 machines already use the net and (kind of) share files. I got PC4, freshly loaded XP Pro, but it can not see the network or use the Net. The light on the router is on, but the router software not see it.
IP detection set for automatic. I loaded this up with 98SE and XP before this week (doing software installation tests) and they were able to use the network before. This next time around it does not.
How can I get this XP machine to use the network?
No file sharing works. I do IPCONFIG and it gives me no info.
Thank you.