This post applies to both Windows 7 and Windows XP (and probably the server OS's as well).
At the office we have two available networks: An internal network with our servers that also has filtered internet and an external wifi network that just offers unfiltered internet.
The main issue is that, as an IT tech, I download a lot of large files that can slow down our network, so I often found myself using a laptop on our external wifi downloading items so that I don't slow internal people down.
My goal was to have two network connections on my Windows 7 PC, one wired internal connection and one a wifi external connection. I also want my internet traffic to go over my wifi connection but at the same time any have the ability to access all the internal sources.
A warning before I start: Doing this incorrectly can open a giant security hole in your network, please don't do it without the approval of your IT dept.
First, I simply hooked up both network connections and tested them both (by only plugging one in at a time and checking the internet) as fully functional.
Once both are functional you need to edit your routing table so the correct requests go over the correct network adapter.
You can do this via the command line, but I found the Nirsoft NetRouteView utility to make this whole process a whole bunch easier.
Download and extract the NetRouteView utility. Then if on Windows 7, right click the NetRouteView.exe file and select "Run as Administrator".
Click the "Interface Name" column title to sort your entries by interface like above.
Depending on your network only one or two changes should be made. First, you need to remove the 0.0.0.0 entry from your internal network. That entry is a "catch all". We want the catch all on the outside network and not the inside. So remove 0.0.0.0 from your internal adapter and make sure its already there on your external connection.
For most people that should be it! But for me I had to add one entry to my internal network to get it to work. I wanted the whole 192.0.0.0 (that means anything that has a 192 for its first number in it's IP address) to go on my internal network connection. So I added the following:
Thats it! To check that everything is going over the correct interface I added the Network Traffic gadget to my Windows 7 desktop twice (just drag it to the desktop twice from the gadget list). I made my internal connection red and my external connection green.
Now I can watch as I view a web page which adapter it's going out.