1) right click 'My Network Places' and click properties
2) a new window will open with your dialup connections in them click the 'Advanced' menu at the top of your screen
3) Then click 'Dial up preferances'
4) Then click the 'call back' tab
5) Then click 'No Callback'
Or if you right click 'incoming connections' and click 'properties' and the 'users' tab if you look in there there will be a list of users allowed to connect to your computer if you double click one make sure no call back is selected.
That should do the trick
let me know how you got on.
If that dont solve it then i aint got a clue