Often the networking options like the USB options can be confusing. With VirtualBox, there are four predefined network cards available. With Windows 7, the Intel Pro/1000 is the most compatible and bridges with your internal cards flawlessly. Depending on the operating system, motherboard or cards that have been installed on the operating system, selecting PCNet or Intel interface almost always insures networking will be functional.
With the options for NAT, Host Adapter and Internal options, each of these can be used to determine which 'network' you want the computer (virtual) to be on. NAT allows for the computer to get an IP address from the host computer. The host computer keeps its IP address and assumes a 10.x.x.x private IP address. The host computer then acts as a bridge to the virtual computer which gets a 10.x.x.x private IP address. This NAT (network address translation) allows the computers to talk to each other and allows the virtual computer to navigate anywhere on your network or the internet. The HOST Adapter setting allows the virtual computer to assume an IP address on the same network as the host computer.
Networking is one of the key features of VirtualBox. Networking allows you to setup patches, service packs and yes, you MUST protect this machine just like the host computer with Antivirus software. Although this computer is virtual, once it is placed on the network, it becomes as vulnerable as any other computer.