Recognising internet connection

I'm Having Issues with one of my Copies of Windows 7, I have it connected to my modem/router. but it's not recognising the internet connection, i have 2 other computers connected, one with XP and the other with XP and they both work fine.
Dose anyone have any tips.
Ry

There have been a few threads about people having issues connecting to the net going to W7. I'm not sure why, if your Xp connects ok and it's on the same home network. Why can W7?

Your modem is also a router, you connect to it wirelessly, and are 100% sure you are on the network (try pinging the other PCs just to make sure). Unless there is some kind of setting in the modem/router (console access via browser) like a MAC filter or something, you should be able to get net access provided it's a fresh build with no alterations to your firewalls etc..

I have a wired connection for all 3 PC's, i just can't figure it out, admittedly i haven't set a home group just because i'm having so many issues with that it's not funny. the computer is able to log into the modem but it dosen't recognise that the modem has the net connection.