yay.....99% of the people playing half life now, use steam, even if this WON2 exists, not many people will play it...
And well, if you previously owned half life, and made a steam account with that cdkey, you do not have to buy the new half life if im right, it should just work....but if it isnt and you need to buy a new half life for the cdkey.....i dont know if it will work, cos steam says "You may only have 1 HL2 package at the time" so this might be the same for HL1 you should really ask it on the steamforums