What you want to disable is MediaSense. This is how M/S says to do it:
1. Start Registry Editor.
2. Locate the following registry subkey:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters
3. Add the following registry entry to the Parameters subkey:
Name: DisableDHCPMediaSense
Data type: REG_DWORD (Boolean)
Value: 1
Note This entry controls the behavior of Media Sensing. By default, Media Sensing events trigger a DHCP client to take an action. For example, when a connect event occurs, the client tries to obtain a lease. When a disconnect event occurs, the client may invalidate the interface and routes. If you set this value data to 1, DHCP clients and non-DHCP clients ignore Media Sensing events.
4. Restart the computer.