If you have the digital distribution version of COD4 dont install the 1.6 patch. It fries the game
They've released a separate patch for direct2drive versions of the game.
In response to Hazman (#1)
Not 100% sure if thats the same thing, its currently downloading, but I did install the digital dist. version. The regular patches wont even let you install unless you edit your registry keys. But anyway, after I installed it now it gives me 2 errors whenever I try to run it. Got my patch from

EDIT: when I try to install that one it says it isnt compatible with mine and to download the patch from wherever I purchased the game, which is what killed it in the first place =P
In response to Falacy (#2)
I did the edit thing for my steam install of COD4 and it broke the single player. All it does is prompt you for the CD when you try to run it. I didn't care though because the multiplayer works fine.