There could be all sorts of reasons for not being able to access the internet from that PC.
Some questions that could help get to the bottom of it:
Is it the same hardware as before the Win7 install?
Did it work before then?
Assuming not wireless, is the cable plugged in to the correct socket? At both ends?
Are other devices connected to that router/modem able to access the internet?
Can you access Device Manager (right-click My Computer>Manage>Device Manager) and see any items with yellow exclamation marks?
Windows Key+R type "cmd" (without quotes), click OK then "ipconfig /all" (without quotes) and press Enter. That should show you various settings related to the network adaptors in the PC. You may need to scroll up the window to see all the details, but you need to know if there's an IP address, Default Gateway and DNS Server for your network connection.