Go into IE, chose Tools -> Internet Options... -> Connections, and tell it not to dial on demand.
This will stop the computer from dialing automatically, which means he wants to use the internet, he will have to connect manually.
Now to find out what is causing the problem. Run 'msconfig' and choose the Startup tab. See if there is an entry that either runs your web browser or a shortcut that points to a website (url). Either of these would cause the browser to open, and the autodial would kick in. Just unselect the offending option and see if it is fixed.
This should solve your problem. If not, then the offending program was really sneaky, and you might have to search the registry. Hopefully this isn't the case
Hope this helps.
I have not yet begun to procrastinate.