WINDOWS XP DOESN’T RECOGNIZE MY MODEM

I had to reboot my laptop (purchased second hand) with a COPY of the WINDOWS XP MEDIA CENTER EDITION start up disk, or whatever they’re called, given to me by the guy I bought the laptop from. Anyways, now xp doesnt recognize my modem. As you can probably tell, I am quite computer illiterate. I am hoping this is enough information to get me back on line. Also, a window keeps showing telling me my Windows is not activated, But everytime I select the activate windows icon in the start menu nothing happens. I have the product key and it was activated before I reinstalled windows, but now it says somethhing about check the crack folder, which I have but I am afraid to touch anymore buttons without knowing what I am doing. It seems I have done enough damage.