some half-way good news...
The professionals tested my GFX card without charging me (they know I am a poor student), said that the card was all in order and gave me a few tips.
Actually it seems ATI cards are --extremely-- picky about what DirectX, graphic driver version and combination of the 2 they work with.
Well... reinstalling the GFX driver several times (!!!!) and DirectX 9 seemed to resolve the problem, but whenever a new hardware is installed which uses direct-draw (like a TV card) or some other multi-media device which uses parts of DirectX the installation gets corrupted again and you have to reinstall DirectX, the GFX drivers or both once more...
But hey I'm happy this shit finally works
Now if I find some more harddrive space and time I might finally install a few games...