, Windows (xp/vista/7) doesnt come with a program that disconnects you from the internet... Also, with msconfig you cant ADD or REMOVE startup items, just disable or enable them.
You can add the batch file to the startup folder... or if you want to be conspicuous, you could add it to the registery. But because i'm not 100% on the REASON for you wanting to do this, I'm going to let you find out how yourself.
You could also just set the subnet of the computer to something outside of your routers subnet... I.E. from 255.255.255.0 to 255.255.0.0
There are lots of ways for you to disable network connections... If your router supports it, you could block the MAC address of the network card that the computer uses.