Thanx for your replies.

@mason wheeler: I don't think the problem has something to do with Vista vs DX9. I compiled and ran my engine dozens of times on this machine without weird errors. I think it's related to my code and propably DLL related.

@Momor: Thanx i'll check that.

I made a stupid mistake. Some days ago i started using Turbo delphi instead of D7. It works great (except for the AV's i get from time to time). So i continued my project and saved my work in different folders V0.12.1.. V0.12.2 etc.
My engine is a DLL. So i develop a testing application and a DLL at the same time. Each time i saved my project into another directory i forgot to set the DLL's host application to the EXE in the new folder.
That way, i was running an obsolete EXE with a newer version of my engine.

No i have to take a few steps back, and search for the last correct version. I'll start coding from there and hopefully i can organize my files properly so i won't make that mistake again.
Maybe i'll bump into this bug again, but when that happens, i know that the last few changes to the source are causing it. Then i can debug it properly.

Until now: I think it has something to do with DLL export functions which have incorrect names/parameters in the APP's source. (i had that problem before).