Snow Leopard killing internet access on home network
Hard to say without being able to look at your network settings in your Mac and in your house in general, but I imagine your MB is creating a network conflict with your internet Router and therefore the other machines (and itself) end up sending the packages to the wrong place.

The first thing that came to my mind was that your MB Pro is not using DHCP and it's using the same IP Address as your Netgear router or Motorola Modem.

Things to diagnose:


Before even turning on your MB Pro, in your Windows machine, check your network settings. Write down your IPAddress, Your Subnet Mask, your Router and DNS Servers if any. Also note if you have DHCP Enabled or you're manually setting your IP.
With the above settings safely saved (on a piece of paper) turn on your MB Pro and check your network settings for your "Airport" -wifi- network: Go to System Preferences, Network and you will see your Airport. Note the numbers and post them back so we can try to see if the problem is this easy or there's something else involved.