Grand Theft Auto IV running slowly

Hello,
I'm wondering why GTA IV is running so full of lag for me. I'm running it on Windows 7 Ultimate 64-bit SP1, on a Intel(R) Core(TM) i7-2600K CPU @ 3.40GHz, with 8GB of DDR3 ram, and I have an ATI Radeon HD 5670 (1gb GDDR5).
It even runs slow when having 800x600 and eveything lowered to none or 0 in the configuration.
Hoping for some answers,
-Launchy21