finally we managed to get the game running !!!!!! thanks for all your help.
installing the omega driver (which took hours...) and deleting this prefprop-file in every game every time did a lot of help...
the games run very very slow though, now in part 3 the sound stutters in the dialogues. thats because its only a 2MHz machine with a 64M geforce4 and 1024mb of ram and lots of free disk space, i know

i've once played some very fast and impressive 3d-games on that same machine in the past (e.g. half life 2).
however the biggest problem now is a bug or 2: i cannot access the options screen, i can click on the button, hear the sound, but the screen won't open. it will SOMETIMES after deleting the prefs.prop (have to exit and delete every time!) but that works only every 10th time (so i don'T try that anymore). so i can't adjust the volume and i get tired from the loud music after a while. and i would like to turn the quality down as much as possible to get a chance of running the game more fluid. another thing is that the game crashes instantly when trying to enter fullscreen, so it's quite tiny on the 1280 screen (the resolution is not changeable in the game!)
i'll put these last problems/questions in another thread so if anyone has an idea of what i can change, youÄre welcome to answer there.
thanks to anyone for your help!