I'm not certain if this will work, but it looks promising:

All of the following items in quotes can be found under Start / Settings / Control Panel

Open "Network connections" and create a shortcut to your dial up connection somewhere convenient (like on your desktop).

Open "Internet Options" and look at stuff under the Connections tab and make sure things like autodial and save username/password are enabled.

Open "Scheduled Tasks" in the control panel and run the Add Scheduled Task wizard. After a list of programs popsup, click the browse button and select the shortcut to your dialup connection (the shortcut you made above).

Indiate to perform this task 'When my computer starts' You may need to enter Administrator credentials. I'd also tell the wizard to open advanced properties upon finish.