I had the same problem. to fix it I had to login to that cruddy grey screen. Then right mouse to get a menu, then there was a shell option (not sure what the option was called XShell or Xterm or something).
Then you can change to superuser with
su
then run Yast with
Yast
When that loads up go into the option System and /etc/sysconfig Editor
Now select Desktop and you will get a load of options about all sorts, and now you should be able to change the display and desktop managers