There is a problem with my PC I bought Gta IV yesterday and I started to play but there is a lot of lag in the game.My PC system=NVIDIA GeForce 9600 GT,RAM 4GB.Windows 7. Any ideas how to fix?

Is your support free?

How many programs, app, and services do you have running in the background of the game? You have to remember that even though you're on the higher end of RAM and a good Graphical Card, this is shared with everything running on your PC not just your game.

Maybe downgrading the quality of the game is your only shot but play with the settings and find out a happy medium for speed and quality with your system.

Make sure your computer is free of any viruses and spyware. Update and run any scanners you have along with Malwarebytes for anti-spyware. If you don't have anti-virus check out Avira or Avast.

After that, run HijackThis and post the log on here.

What sort of CPU you have?

Intel(R) Core(TM)2 Due CPU T6400 @ 2.00GHz 2.00GHz

Did you even check your background processes? CPU won't matter because you're well within the requirements for that game.

CPU does matter! Because the requirements of this game is:

GTA 4 Minimum System Requirements

* Processor: Intel Core 2 Duo 1.8Ghz or equivalent AMD Athlon X2 64 2.4Ghz
* Memory/RAM: 1GB for XP and 1.5GB for Vista
* Video Card: 256MB NVIDIA 7900 or 256MB ATI X1900
* Hard Disk Space: 16GB free
* Operating System: Windows XP SP3 or Windows Vista SP!

I have GTA 4 too its so lagging as well, allot of people have problems with this game even with the recommended requirements.
but there is away to solve this watch this:

http://www.youtube.com/watch?v=Ek6RamLu_WA
http://www.youtube.com/watch?v=boqjdbxfoSs