you're right, very stupid of me, sorry.
i failed to notice you have to make nero show "image files (*.nrg, *.iso, *.cue)" instead of "all nero compilations and images". and because i couldn't see the file, i thought i had to unpack it... well... it's really good that being stupid doesn't kill instantly
now ubuntu starts very well
but actually, there's another problem...
after i came back to windows, it couldn't see the network card and the printer drivers
i managed to get over it by rebooting it and rebooting once again during the windows start-up, and than choosing to "run the last good configuration", but i can't resist the feeling this is not quite the way it should be...
another point is that the sound card didn't work under ubuntu. but this isn't not really important.