Well ive had the same sort of problem as described but on windows XP, and it was solved by the automatic metric calculation answered, thanks kevin
I will explain my setup and exactly how I solved it.
My computer is connected to two networks, the first is a wifi card to a router for internet access, the second is by a wired network card attached to a hub. a temporary meassure put in place so I can configure a NAS box also attached to the hub.
My predicament - not being able to not browse the intruction manual online whilst I configure the NAS box through its own web based interface!
I set up the following subnets
192.168.100.1 - wired to the NAS box through a hub
192.168.200.1 - wifi to the internet through a router
The effect was strange, sometimes I could browse a page other times it would just time out, clearly internet traffic was getting lost down the wrong subnet.
Heres how to fix it..
Open up a command prompt and type 'route print', you can then verfiy the 'metric' for each subnet your running, look at the lines where the Netmask shows as 0.0.0.0, take a note of the metrics, the wired network will most likely be '20' and the wireless '25', note: the lower value tells your computer to use to use that subnet over the other, certainly in the case of web browsing.
Go ... Start menu > Control Panel > Network Connections > Open the Properties of the non-internet network > under the 'general' tab and from within the 'This connection uses the following items' list, select 'Internet Protocol (TCIP/IP)' and click the 'Properites' button > From under the 'General' tab click the 'Advanced' button > under the 'IP Settings' tab untick the 'Automatic metric' box and input into the 'Interface Metric:' field the higher of the two values collected from the 'route print' command.
Repeat again for the internet enabled subnet, but this time entering the lower value, you can then verify the settings by going back and running the 'route print' command again.
Hope the intructions helps somebody