Nexus is the hauppauge branded version, skystar 1 is the technisat, of course they're all really technotrends
Support exists for XP and Vista, of course. THe only advantage of the card is that it has a hardware mpeg2 decoder which was badly needed back in the days when PCs weren't capable of
that is called "hardware mode" and it displays an interlaced image off the tv-out connector. The skystar 2 is a software only card, and thus obviously doesn't have tv-out. You should notice that the Nexus/ss1/whatever is also capable of working in software mode.