If Windows 7 couldn't detect your router how could it ever connect to one automatically?
If you slap any piece of DHCP (Dynamic Host Configuration Protocol) compatible hardware on the end of an ethernet cable connected to a Windows 7 DHCP enabled network adapter then I'd imagine you'd expect the operating system to see it wouldn't you?
For this reason most routers are set to have an active DHCP server function; this automatically allocates a pool of IP addresses to anything joining or rejoining your LAN/VLAN/WLAN (or your private intranet, or network, in other words).
DHCP basically allows a computer to join an IP-based network without having a pre-configured IP address. DHCP assigns unique IP addresses to devices, then releases and renews these addresses as devices are powered up, or re-join the network.
Network equipment therefore offers DHCP support for added convenience in joining computers to the LAN. It saves you the laborious task of being obliged to manually assign a subnet and IP address to everything on your network, every time it's connected to the network.
To protect your network devices there is therefore also a firewall in routers that will not allow anyone on the WAN'side to see anything on your intranet.