Are all games on GOG.com able to work on Windows 7? I'm wondering because a few titles in the gameslist only says XP and Vista.

Officially, not all GOG titles are Win 7 compatible (they haven't finished testing everything), but I think you will find that if a game worked on Vista, it will work on 7.

There is a topic about what games run in XP and 7, I reccomend you give a look.

Well, I used to have XP on my computer, but for a special reason I had to change to Vista, but the games that I have tried to run(Simon 2 and Tyrian 2000) only show in the middle of the screen, really reduced screensize. I've also hade a few other problems with Vista as well.

So I was thinking of either changeing back to XP, or perhaps upgrade to Windows 7. But if some games don't work on Windows 7, i'll go with XP.

The ones with Win7 listed have been officially tested. That doesn't mean the rest won't work. There are only a few games with significant Win7 issues.

That's not a Vista or Win 7 problem, that's either a video drivers configuration problem, or a DOSBox configuration problem (your video scaling is set wrong). The same thing should happen on XP, assuming that you have everything configured the same there.

"Disciples - Sacred Lands" runs fine on my Vista desktop PC but won't even start on my Win7 netbook. Of course the reason for that may not lie in Win7 alone or at all but fact is that this is the first game from GOG that I can't run on my netbook setup (might be because it's neither using Dosbox nor ScummVM but e.g. Syberia, Age of Wonders, Temple of Elemental Evil or Neverwinter Nights are all Windows based, too, and they run without issues on the netbook).