Well, I know that it can be done through linux, so I can only assume that it's just as easy through windows. do you have a gateway/router that's also connected through your configuration? If so, why not block yourself out from using the internet until the hours between 2 and 8.
See, i believe that what you are going to try and do won't work very well considering the DSL modem that you're using. it's probably designed to use similar scripting to reconnect in case of a failure. The best thing for you to do is look at some of the technical documentation on the type of modem you have and how it uses it's ppp-on, ppp-off and reconnect scripts.