I had a duel boot system, FC8 and Win Vista, running grub as the boot loader.

I upgrated using the DVD of FC9, which all went OK. (this is on a Dell Latitude 
D830 laptop).

So then I wanted top try KDE, because the package manager on Gnome wasn't 
working for me). Googled, found i needed switchdesk.

yum install switchdesk

then

switchdesk "KDE"

something like 
> Cannot use KDE, KDE not installed. Download "KDE GROUP" blah blah.

yum install "KDE-group" (or the exact  line it told me to install.

KDE installed about 125MB.
Ran:

switchdesk "KDE"
> OK.

 and then I rebooted.

Grub failed and printed "GRUB" exactly 6 times on the screen and froze.

Nothing worked.

The only wy to recover was to put the FC9 disk in and do a repair and reconfig 
of the bootloader.