Are the computers (especially the Windows 7 machine) connected wirelessly or through an ethernet cable (wired)?

If connecting wirelessly, did you make sure the wireless adapter is turned ON on the computer? (Usually a button which needs pushing.)

Did you make sure the router is setup so the SSID is being displayed and the security password is set as WPA/WPA2? If not, please do.

In the meantime, have you tried re-initializing the modem/router by following the instructions below?

Turn OFF all the computers, then turn OFF the router, then turn OFF the modem (if it's separate from the router). Wait 10 minutes. Then turn ON the modem, wait two minutes for it to re-initialize with the ISP, then turn ON the router (if it's separate from the modem), wait two minutes, then turn ON the computers.

Hope this helps.