So we all know how bad the PC port of GTA IV is... It's not using system resources as it is supposed to and is running at a very low framerates for a lot of people, even with good enough system specification.

In case you set it to lowest graphics possible and still not satisfied with the framerates, follow this guide and you may notice a big improvement.

First off, If you have Nvidia, download the latest driver. It was released specifically to enhance GTAIV's performance (181.00):
XP
Vista

Disable bloom effect and blur:
you can't do that in-game, you have to configure it manually.
Thanks to the heavy layers of protection incorporated into the game, any change to the game files will terminate the game on start up. So you have to trick the game into thinking that the files are unchanged.
Follow these instructions carefully:
1) download magic.bat
2) Move it to your GTA IV main directory.
3) run magic.bat - This batch will allow you to modify the game files in order to change settings without causing the game to terminate on start up.
4) navigate to "Grand Theft Auto IV\common\data\" folder and locate "visualSettings.dat"
5) open it with wordpad in order to change graphics settings.

/* Any thing you do from this point must be done carefully - make backups of the files you edit to revert back in case something went wrong*/
To disable bloom and blur:
scroll through "visualSettings.dat" until you find "#Config Values for Camera Events Val start Val End MB start MB End"
change all the values to zeros. if the value is 10.00 change it to 00.00 and so on... (this will disable blur with camera movements)
Then you'll find "# Misc Values"
Same as above, change all the values to zero EXCEPT FOR THESE LINES (keep them as they are):

save the file.

Now, after disabling blur and bloom effects, you might want to force-disable vertical sync:
1) create a txt file and name it "commandline.txt"
2) paste the following:

Save it and move it to your GTA IV directory (same directory with GTAIV.exe)
This will disable memory restrictions, Vsync, and precache of resources.

Still not satisfied with the framerates? want to set it to absolute lowest quality?
add these lines to your "commandline.txt":


Save... Play... Enjoy!

After you are sure the game is running fast and smooth enough, start modifying the in-game graphics settings and increase resolution, texture details, etc as long as it doesn't cause any lag.

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// EDIT::
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Extra notes:
- The game might lag after continuous playing, as well as a missing textures bug... to fix this (without having to restart the game) go to graphics options, change the resolution to whatever, then when it asks you if you want to keep it, hit no to return to your original resolution... The game somehow "refreshes" itself; the missing textures will reappear and there will be no lag anymore.

- If your PC is really old to handle the game, you can still play it with decent FPS if you disable shadows, reflections, etc...
This can be done be applying the GTA IV asi loader.
Download here: IV asi
Move all the files to the GTAIV directory (make backups of the original files first)... Open up flags.ini and modify the flags.
Band #19 is the shadows band (1=enabled, 0=disabled).
ie: to disable shadow, paste the following flags and save: 11111111111111111101111111111111
each band represent something, they are all enabled by default. Unfortunately, I don't know what each band represent so you can try disabling some and checking the game for any difference.
You can modify flags.ini *while* the game is running without having to restart, and it will still take effect.

Enjoy :)

Thanks Bolshoi