Residential Wireless Devices such as wireless B and G and even N routers use a public frequency to broadcast their information i.e 2.4GHz or 5.4GHz this means that ANYONE can use it either with their Wireless Router or if you have a mad scientist down the road trying to setup his death ray on the 2.4GHZ band this will also interfere with your wireless connection. (ie slow down in internet connection or low signal strength)
In the case of a mad scientist, there will obviously be no visible "Wireless Network" so to say, so what you need to do is start of with the 1st channel on your router and test it, then go to the 2nd and then 3rd and so on, you should eventually find a channel thats clean enough for you to use.
In the public Licensed Wireless Frequency each band has whats called a guard band, a small band where no transmission of information or broadcasting is allowed, unfortunately the likes of the 2.4GHz and 5.4GHz don't have these guard bands so its just a matter of finding the cleanest. Or my other suggestion would be N range of routers. The only thing you have to be careful of here is N is still in draft and has some issues make sure 100% that both your Access Point (Router) and your Access Device (Wireless NIC) are compatible.
2.4GHz Band
Channel 1 50% dirty including 10% leakage into channel 2
Channel 2 70% dirty + 10% leakage from channel 1 including 1% leakage into channel 3
Channel 3 10 % dirty + 1% leakage from channel 2 including 8% leakage into channel 4
Channel 4 90 % dirty + 8% leakage from channel 3 including 60% leakage into channel 5
Channel 5 40 % dirty + 60% leakage from channel 4 including 15% leakage into channel 6
Channel 6 10 % dirty + 15% leakage from channel 5 including 9% leakage into channel 7
In the case above you could be setup to use channel 4 and therefore flooded with dirty air, while if you swapped to 3 (coincidence thats its neighboring channel) you get only 11% dirty air and a clean connection.
Thats how the wireless part of your network works as for the wired, see the nice red\green\blue\orange\yellow or whatever color the ethernet cable is, that insulates the cables and protects it from interference and noise.
My suggestion above should be taken into consideration before you decide to buy a new router or WNIC as if you come along and setup a new piece of equipment and the only true issue was dirty air then you are down a few bob.
Hope this helps and answers some of your questions
