Then this is likely a driver issue, about which we can't do very much. Considering the odd resolutions you see I wouldn't be surprised (as you have mentioned) that it is related to the taskbar. Also - is this a multi-screen setup? If so, perhaps try using only a single screen.
I have basically the same windows 7 version (though build 1601 - which I think is one service pack?), and it works for me (though on a laptop with 1280x800 only), and I don't have any problems with the task bar. Perhaps you can try to disable the taskbar and see if this makes a difference?
There doesn't appear much we can do about this problem, it's either an irrlicht problem or a driver/windows issue. Perhaps you could try another irrlicht program - the library comes with examples precompiled and 1 or 2 of them come with a fullscreen option. Just try starting the Demo in irrlicht-1.8/bin/Win32-VisualStudio, select fullscreen, and see what happens when you press start.
Cheers,
Joerg