Firewalls do this. Either Windows or third party firewalls can prevent local area network access if you don't visit their settings to allow such.
HI. I recently came back to cable modem, and set up my linksys router. hardwire to desktop, wireless to laptop. internet works great on both, but can not share. checked the share settings on both, checked computer and workgroup name, and no luck. I am not real savvy on computer knowledge but open to try. By the way, this used to work when it was initially hooked up.