I'm like the rain on your parade tonight... I'm sorry about all this. =/
Halo 2 is XBox exclusive. Unlike Halo 1, where Bungie promised a PC port one way or the other, Halo 2 isn't being ported. Really, Halo's PC port played just like the Xbox game, down to the response of the controls, so there isn't really a reason to hold out hope for a PC port, IMO.
If you've got an Xbox, though, I recommend Ninja Gaiden. It's not appropriate for your son, unless you unlock the NES versions hidden in the game (Ninja Gaiden II--best action game ever.) Very hard, very well-put-together. Might change your mind about consoles. And if not, well, at least you'll have 20 hours of fun.
Dunno about the rest of the world, but from here it sure looks like Steam has folded - at least momentarily - under pressure. Had a feeling this might happen ;)