How does a Windows computer behave when it has multiple default gateways?

If Windows has multiple interfaces connected to several networks that use DHCP, it installs default gateways for all those interfaces.

By default, Windows sets the metric of the default gateways to the same, effectively leading to random selection of default gateway.

If you want to make Windows to select certain gateway always, you can change interface metrics for the interfaces' configuration you don't want to use:

1. Go to your Network adapter properties
2. Select Internet Protocol v4.
3. Click the Advanced button on General tab.
4. Uncheck Automatic metric.
5. Enter a number higher than 10 to the Interface metric field.

The default automatic metric for interfaces is 10. The lower the metric, the higher the preference to use it. So, you have to enter metric higher than 10 for the interfaces you don't want to use.

EDIT: These instructions are for Windows 7, XP might be different. You shouldn't be using XP anyway.

1. Your computer will not apply DHCP configuration from BOTH DHCP servers at the same time.
2. Windows computer CAN have two default gateways. You cannot set second default gateway in GUI, but you can add it in command line with route command. In this case Windows will use path with lower metric value.

You can check if you have two default gateways with route print command. Default gateway is there, where both network address and network mask are assigned to 0.0.0.0