OK, I bought this.
Counter Strike Source Multiplayer Pack, see.
And it installed Steam and everything like it's supposed to.
And it plays fine for 2 weeks or so.
Then whenever I try to launch the program, it says "Did you like the free Day or Defeat weekend? If so, you can buy the full thing here".
But I've already bought the full thing!
Now CS:S won't even launch. I click it and it says "launching game..." Then the screen turns black and and then I'm at my desktop again without me running CSS.