[CentOS] Can't login after installing Centos 5.5

I've done a clean install of Centos 5.5, largely using the default
options. Those I changed were:
* Changed to KDE Desktop
* Added Samba and NFS4 to firewall
* Changed NTP servers to those of my ISP

When Centos boots what I think is a progress bar shows no progress but
eventually (probably about the right amount of time for booting the
OS) the user login screen partially appears (username and box, time
and name of machine, a flowing "tree" symbol), but the bottom of the
screen does not appear to be completed. This login screen clears and
redisplays several times.

Finally the screen locks with the time displayed in much exaggerated
height (about a quarter of the screen height) and nothing more works.

At no point can I get to a text login screen (Ctrl-Alt-F1) so I can't
even debug it!

Can anyone suggest what is wrong or how to overcome it?