So when you start the computer and boot to Mandriva, does it fail to start the X server or does it go to the regular login screen (graphical) and when you login, thats when you see the gnome splash screen and then X crashes? If so, just choose KDE from the login screen and try to fix gnome that way. You may be missing something like metacity or GTK2.x so try and and check the dependancies.
You can't start gnome in safemode, safemode in Linux is the command line.