S.T.A.L.K.E.R backup glitches

[cptpooface]

I successfully started stalker using the S.T.A.L.K.E.R. v1.0003 [MULTI4] No-DVD/Fixed EXE from game copy world. But I am still having a lot of problems with it. I can get to the main menu, but then about 50% of the time I will start a new game or a load a game and will get stuck at the load screen, when this happens I have to close the game and try again. When I actually do get into the game it is very laggy. For example, I was at the point of the game where I had to get across the train tracks, either by paying the military guards, or going through the other tunnel. I load the game and it takes me forever just to move two steps. After a while, the area will become less laggy, almost regular, but will still lag really bad every once in a while. Other times I might be aiming at a mutated dog and pull the trigger, then the game will lock up, sometimes for a long time sometimes just for a few seconds, but I can never play the game lag free or without it freezing up randomly. I am also noticing very very long load times. Does this happen to anybody else? If it does happen, is there anything I can do? I have already patched the game to version 1.0003, which is the same version number of the crack. My computer runs on a intel core 2 duo 6600, an Nvidia Nforce 680i SLI motherboard, a Nvidia 8800 GTX video card and 1GB of RAM. So if anybody can help me out I would really appreciate it.