Cannot login after updating yesterday

Hi, I've been running FC4 test 2, and figured that with FC4 release imminent things should be pretty stable by now, so yesterday I ran up2date to update everything, initially bar the Kernel. Mostly went OK but there were a few problems afterwards: - GNOME desktop icons had vanished and I couldn't find a way to recreate them. - Right-click on GNOME desktop didn't bring up usual menu. - I couldn't login to CUPS. Then I decided to update the remaining items (kernel, kernel-devel, etc.) too. This update went OK except that up2date hung at the end of the installation phase. On re-running it, it told me everything was up to date so I rebooted. Now, with the new Kernel, I cannot login in at all. Trying to login as root or another user gives an error "no shell" and then back to the login prompt. Is there any way to get round this other than a full re-install? Ian