Had the same problem recently, but have solved it. In my case, it was down to the actual modem (which never did explain the whole desktop/laptop thing).
If the modem is the white 'Webstar' model, then try this link...
http://www.onemanblogs.co.uk/index.p...ar-cable-modem
Worked a treat for me but appreciate that it was a particular model issue in my case.