Tip 3: Virtual LANs and Wireless "Privacy"
Consumer networking gear is getting more sophisticated with features appearing that previously were only available in "enterprise" grade equipment. Although it's unlikely, your router could have Virtual LAN (VLAN) capability that could prevent file sharing from working.
A primary purpose of VLANs is to allow computers on different physical networks to behave as though they were all on the same network. But that same capability can also be used to isolate computers attached to the same router or switch so that they can't communicate with each other.
Unless your router (or switch) specifically states that it has VLAN capability (and provides a means to control it) you can safely assume it's not VLAN capable. If your networking gear is VLAN-enabled, then dig into the manual and find out where the settings are and make sure that all the settings are cleared.
Wireless Access Points and routers, however, are a different story. Some products implement a "wireless privacy" feature (it's also known by other names) that allows wireless stations to connect to the wired LAN, but not to another wireless client. If wireless client-to-client file sharing isn't working, check your access point or wireless router's settings carefully and look for this setting.
Wireless privacy settings are not called "Block SSID", MAC address filter, or Access Control List (ACL).