When buying Valve video games, do not use the oh-so-convenient steam app to download and install them directly. Buy them from the store, just like everyone else. If, for whatever reason*, they decide your account has been used for cheating and de-activate it, it is much more difficult to get things back in order if you don't have a CD key. As in, effectively impossible.
And I was having such a good time re-playing Portal, too.
* I personally think it was some sort of bizarre interaction between Vista and the steam app, since my own troubles started just after a big honking Vista crash, and renewed after a steam update. I just don't have the time to play games online anymore, and have never seen the point of cheating even when I did.