• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RasMan\PPP\EAP\25
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RasMan\PPP\EAP\26

Tidy up any other registry keys (eg Juniper) and reboot.

From kb239924:

SUMMARY

On a Windows-based computer that uses TCP/IP, you can use the Media Sensing feature to detect whether the network media are in a link state. Ethernet network adapters and hubs typically have a "link" light that indicates the connection status. This status is the same condition that Windows interprets as a link state. Whenever Windows detects a "down" state, it removes the bound protocols from that adapter until it is detected as "up" again. Sometimes, you may not want the network adapter to detect this state. You can set this configuration by modifying the registry.

Note 10B2 coaxial (RG-58) Ethernet cable is not a connection-based medium. Therefore, Windows does not try to detect a link state when this kind of cabling is used.
MORE INFORMATION
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

To prevent the network adapter from detecting a link state, follow these steps.

Note The NetBEUI protocol and the IPX protocol do not support Media Sensing.

   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.

Note Microsoft Windows Server 2003 supports Media Sensing when it is used in a server cluster environment. By default, however, Media Sensing is disabled in a Windows Server 2003-based server cluster, and the DisableDHCPMediaSense registry entry has no effect. In Windows Server 2003 Service Pack 1 (SP1), the DisableClusSvcMediaSense registry entry was introduced. You can use this registry entry to enable Media Sensing on the Windows Server 2003-based nodes of a server cluster.

The details of the DisableClusSvcMediaSense registry entry are as follows:

Key: HKEY_LOCAL_MACHINE\Cluster\Parameters
Name: DisableClusSvcMediaSense
Data type: REG_DWORD (Boolean)
Default value: 0

By default, the DisableClusSvcMediaSense entry is set to 0. When this entry is set to 0, Media Sensing is disabled. If you set the DisableClusSvcMediaSense entry to 1, you can use the DisableDHCPMediaSense entry to enable Media Sensing. This behavior matches the behavior of a Microsoft Windows 2000 Server cluster environment.