Valve should not be selling anything without clear system requirements.
All of their games should state:
OS: Windows® XP or higher
or something to that effect, which means XP, Vista and Windows 7. If it only lists a single OS, that's all they offer support for.
They're usually pretty good about this, if a game won't run on XP (for exmaple, it needs DX10 and has no DX9 support) then they usually have a fairly clear and bold notice, like this dog and pony show of a game (just don't read the game credits):
Another good example using that game, it does not list Windows 7. The game does actually work jsut fine with Windows 7 (oddly enough I have a copy of that game), but there's absolutely no support for it on Win7 - the company that made the game (The Creative Assembly - Australia) no longer exists and the publishers (Sega) have no desire to support the product on Windows 7, so it's not listed.
Generally though, if something works on Vista it really should also work on Windows 7. If it only lists XP and not Vista or Win 7 I'd be a little more wary.