Help for running gta4

Hello,
As per the requirements of GTA4 game , my laptop is equipped with everything just except my clock speed is not that much.My laptop's clock speed is 1.67GHz and for the game I need 1.8. Whenever I try to run that game it doesnt runs smoothly, it always lags. Iam running it on windows vista with 2gb RAM with NVidia 8400 series with 256mb dedicated video memory and 50 GB of remaining space. Will replacing it with windows XP help?? Help me to come out of this problem. Is there any other way I can run it smoothely?
Pls let me know as soon as possible.
thank u vry much..