dans la doc de RASPPPoE
pas testé mais il parait que ca marche bien
5.6 Making a dial-up connection "always on"
Users who enjoy flat rate Internet access may find it desirable to turn their connection into an "always on" connection that is established once when the machine boots (before any user logs in) and kept until the machine is shut down. To make your dial-up connection "always on", follow these steps:
If your service provider requires authentication, make sure you have saved the password by checking the Save Password checkbox in the Connect Connection Name window and connecting at least once.
Right-click the My Network Places icon on your desktop and select Properties to bring up the Network and Dial-up Connections window.
Locate the Dial-up connection you created for PPP over Ethernet, right-click it and select Properties.
Select the Options tab and clear all checkboxes under Dialing options.
Under Redialing options, set Idle time before hanging up: to never and check the Redial if line is dropped checkbox.
Click OK to save the changes.
Now click the Start button, select Settings then Control Panel to open the Control Panel window.
In the Control Panel window, double-click Scheduled Tasks.
In the Scheduled Tasks window, double-click Add Scheduled Task.
On the welcome screen of the Scheduled Task Wizard, click Next.
At the program selection step, click Browse... and browse to your WINNT\System32 directory.
Type RASPHONE.EXE (note the spelling!) in the File name: edit box or locate it in the directory and select it and click Open.
Make up a name for this task and under Perform this task: select When my computer starts. Click Next.
Enter your password. Note: The task must be run under the same account which the dial-up entry was created under.
At the final step, make sure that Open advanced properties for this task when I click finish is checked and click Finish.
In the advanced properties, edit the Run: edit box and append the command-line parameters " -d "Connection Name"".
Go to the Settings tab and clear all checkboxes on that page.
Click OK to close the task's properties.
Finally, you need to make a little registry change to prevent Windows from disconnecting when a user logs on and off again:
Run REGEDIT and navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Then right-click the right-hand pane, select New -> String Value, name the value KeepRasConnections and set it to 1.
Reboot. Windows will establish the connection automatically and keep it until you shut the machine down.
NOTE: The connection will not be properly terminated when shutting the machine down or rebooting. This can cause problems with service providers who take very long to detect such a dropped connection and limit the number of concurrent connections. See Known Issues for further details.
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savent po faire une recherche sous google... 
