Bad condition of GTA 4

Hello,
My pc is core 2 duo and 512mb 9400gt. But the game is running slow during night time in gta4, even though the game has low level configuration it is stucking up and even my computer is windows xp sp3 and indeed i have upgraded from sp2 to sp3 for gta4 itself. but also it is stucking up. so please give good results as possible.
There is no virus in my computer

Thank you very very very much for your future results.

.Give me answers as soon as possible.