My system has died when I switched from the default GNOME desktop to the KDE desktop. The process I used was:

1 - Logged out of GNOME.
2 - Clicked on Session button at the bottom of the login screen and then changed the desktop choice to KDE.
3 - Then I tried to login and was dumped out to a blue message box. The box was blank except for two buttons: Yes or No.
4 - Pressing Yes dumped me out to the command line interface.
5 - I cannot reboot successfully or get back into the GUI at all.

Something must have stepped on top of something else and corrupted some settings. This has happened on several machines. Any suggestions on how to fix or prevent this in the future would be greatly appreciated.

Many thanks!

If this happened on several machines, then maybe your install media is corrupt. Did you check the md5sum of the iso's before you burnt them (assuming you downloaded them)?

Baldrick

Baldrick is right, try to get different CDs. If it happens in the same place it's probably the CDs. Ohh, and make sure you get a different version. No point in downloading it, if the actualy ISO image is bad.
Check the distro's bug list too, you can find that from most of their websites (under bugs, or bugtaq)