The default settings make it easy for any neighbor or passing stranger in range to connect to your wireless network.
Your wireless, by default, broadcasts the SSID (Wireless network name) on the airwaves. Anyone turning on their PC will see that and connect. Here is how to prevent then connecting:
1) Change the SSID on your access point and your PCs
2) Set the access point to NOT broadcast the SSID (at this point, a neighbor would need to know the SSID or have special equipment to detect it). For you to use it may require you to maunally set up your PC wireless properties to add the wireless SSID to your preferred networks.
3) Use the WEP encryption; 128 bit is best, or use the best that all your equipment supports (If PC support 64 bit and also access point, then use that)
At this point, you should have prevented neighborly access. Your firewall and antivirus on you personal computer does prevent people from accessing on your computer, but does not have any affect of whether people in the neighborhood can attach to your wireless network.
BTW, if you still have concerns, it is also possible for some router to set up a restriction to only allow certain network card addresses (MAC addresses) to attach to you network. That is likely overkill for you.
Other advice, on your wireless router, change the default password from "admin" to something secure for you to use...remember it.