Oh hell, don't even get me started on this one... I have a Hauupauge PCI card too, and the only way to get it to work properly was to arse around with the drivers until it recognised the card properly. Then, in the Display->Configure (or Preferences, can't remember), there's an option for forcing it to switch resolutions when going fullscreen. Also, the Hauppauge WinTV=>Primary Start Menu entry will let you switch between rendering methods, fiddle around a bit- works differently for everyone because of the diversity of graphics cards.
Failing that, give them a ring, they're actually quite useful and helpful (weird, that), and should be able to help you out.
Dopefiend