The wireless broadcast has a limited range, how far depends on which standard. In order to protect yourself from casual logging on to your wireless network there are a couple of things you can do. 1. Stop the wireless router broadcasting it's SSID. 2. Turn on WEP encription. WEP is not the most secure but it would still take a pretty determined individual a lot of time monitoring your wireless network to crack it. Unlikely for a home user. Better still, if both ends of your setup support it, turn on WPA. 3. Turn off the router when you go to bed/are out. 4. Another thing you could do is limit the wireless connection to include only the mac addresses of your own kit.
I would not worry too much if I were you.
Note that the router will not stop a virus, only an AV program will do that. The router has a firewall that stops unauthorised entry to your PC/network.