Connectin second XP home comp to broadband

My friend has two comps and they are connected to eachother and the other one to the adsl modem. My question is, why does he have to restart if he wants to connect to the internet and the other, the directly connected, computer was booted afterwards?

 

He doesn't. But he will need to refresh the client's settings. Exact method depends on the operating system so if you could say what, someone can say how.

 

XP. Will it be enough to log off and back on?

 

With XP it's even easier than that. Assuming he only has a single network card

start~run~cmd
ipconfig /renew

For more, just do ipconfig /? to get details.