Setting Default Network Adapter in Windows

If you have multiple network adaptor, for example network adaptor and Lan adaptor and you want to change the default adaptor which is used by windows follow the instruction:

you have to manually add a "metric" to each interface. Windows will automatically use the interface with a lower metric. To check and change your network adapters' metric:

1. Open Command Prompt and type: route print - you will see a list of active routes, the last column displaying their "metric". Lower metric routes are preferred over higher ones.
2. Open the Network Adapter Properties (Control Panel > Network and Internet > Network Connections > right-click on adapter and choose Properties)
3. Open the properties of Internet Protocol Version 4 (TCP/IPv4).
4. Click on Advanced.
5. Untick "Automatic Metric" and set the interface metric to a number.
6. Hit OK until you close the Network Adapter properties.
7. Repeat steps 2-6 for your other network adapter(s) choosing different metrics. Remember lower metrics are preferred over higher ones.

for more information you can read:
http://support.microsoft.com/kb/299540
http://technet.microsoft.com/en-us/library/cc771274.aspx