GTA IV Mega Lag Urgent

Hi,
I've bought GTA IV a few days ago and now i install it, the problem is that when the installation finish and that i launch the game, the game lags even at low details. I don't Know why!! please help me!!


Config: Windows XP SP3
1.5Gb of Ram
Intel celeron 64 2.66 GHz
Nvidia GeForce 7600GT 256mb DDR3



Thanks in advance for replies :)