You should read the Rockstar’s GTA IV PC support site, if you are having problems or want to try to improve the games performance.
If you are wondering why you can’t set the game to high detail, want to know what the graphics settings were on the consoles or want to know just what to expect from the graphics auto detection then read the GTA IV graphics settings document.
Read this document for a description of the GTA IV error codes.
The game is awesome, but the graphics are making it look less nice and it lags a bit which is horrible. I hope they can fix it.