I'm thankfull for your help
I have retrieved a full package and documentation that came with a computer (the computer is not mine, I'm was just asked and begged to get it running), and I have dicovered the following:
There is an aditional SPDIF/SV/HDMI output card that was not installed inside, and is labeled by mobo manufacturer, with the same mobo model (ASUS M2N-VM HDMI). I'll try installing that, maybe this would help.
Yeah, I know it works fine under vista, I use it on my own PC