I don't know, sorry.
But if the key's invalid, maybe you could call customer support and ask for a new one
but if the game you bought looks fake, then you were ripped off.
You still might be able to get a new key though.
Almost definitely if the product you bought is legit except the key which could
have been already used or was a mistake or something.
Also check the key: Ds (letter D) and Os (letter O) might be 0s (number zero)
and vice versa and
that kinda stuff. Also, check if the game is allowed through your firewall
and give it full internet access.
Also look on Google for solutions to 'invalid GTA IV key' or something similar.
Hope that helps.
"Sometimes you gotta run before you can walk." - Tony Stark, Iron Man.