Uli
Since you already have solved the problem it may be irrelevant to you anymore, but let me inform you this anyway if this helps.
For Windows Vista or 7, you must disable PCI-Express power saving option to let the installer to upgrade the firmware. This would become an issue if your computer does not have anything other than Intensity (no VGA, sound cards, anything) so that your PCI-Express would remain idle unless you touch this option.
Regards