If you have the VirtualBox host network enabled, LAN worlds do not broadcast on your main network, causing other people on your LAN to be unable to see them.
A temporary fix is to disable the host network in the Windows 7 wireless settings.
Comments 4
Minecraft has no way of knowing what network is the "correct" one. That is something determined by the operating system.
What's correct and what's not correct cannot be decided by Minecraft. If you have to go to from the VirtualBox Network to the LAN over a gateway then Minecraft won't broadcast to LAN.
Anyway, this is a technical support issue; this site is for bug reports only. For technical support please use the Mojang Support Center.
That's not a Minecraft issue.