I try to run GTA IV on my PC and right before I start playing it asks for my GFWL sign-in information. I put the correct credentials in and It then asks for my product key. I put it in and it says it is invalid. I have tried it many times and it will not work. I have a legitimate disc of GTA IV and the product key is still invalid. Please help. Thank you.                   - Dark_Gap

You should probably contact Rockstar or the retailer you bought the game from and get a new copy and/or serial number.

Is it the first time you're installing the game, I mean, did you bought it recently ?

If yes, it may sound stupid, but be sure to type the correct characters, because sometimes they are barely readable (0 and O, 8 and B, etc...).

 

If it's not the problem, do what "Someone22" said, contact Rockstar (or the retailer, but sometimes they don't care lol).