Witcher 2 lagg/low fps?

My specs are: Windows 7 Ultimate 32bit, Intel Core i7 920 @ 2.67 - 2.7 GHz (8 CPUs), 6144mb of RAM, ATI Radeon HD 4870 with approx. total memory of 1781mb. With Witcher 2, I can only get up to 20-30+ FPS on medium-somewhat high setting (Tex. memory size - medium, VSync - off, Dangling obj. - off, UberSampling - off, LOD Distance - minimal). Why is it so? And how do I fix it? I would like to play the game with 60+ FPS but also with highest settings possible. I mean, I don't know much about hardware, but I think that my PC is somewhat good, no? Same with GTAIV (30+ FPS) and even Witcher 1 (which is like a game of 2007), with highest settings (even disabling VSync and anti-aliasing). My rig isn't custom, it's a factory Acer Aspire 7720.