GTA IV lagging

Hello,I have AMD Athlon 64 X2 Dual Core 6000+ working 3.0 GHz 4GB ram 8800 GT 512 MB vram.My problem is GTA IV is lagging,i mean when I play play game it looks like I ha ve to slow PC for it.I put all settings in video to low even resolution at 800x600 and game is still lagging,if you know what I mean.Sry for my english but i trying to explane my problem :) I didnt have any problems before with GTA IV i played at maximum resolution 1600x1050 i mean not sure all details at high and game worked very good not single problem.Now the game went to hell dunno whats the problem I am useing XP SP3.If anyone got any idea whats wrong and know how to fix it pls tell me.You can answer me in my email its Vucko90@gmail.com.Thank you very much