|Thats the problem with using task packages.
task-x-window-system depends on xdm, ximian-gnome I assume depends on gdm, xdm and gdm cannot be installed at the same time (it doesn’t make sense to), so therefore task-x-window-system and ximian-gnome cannot be installed at the same time.
Your best bet is to remove task-x-window-system, it’s only a meta package anyway, and not doing anything important.
The ximian-gnome package being broken is another problem. I tend to agree with the post above about steering clear of the ximian packages, but the ones in stable are supposed to work ok.