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.
*** This bug has been marked as a duplicate of bug 450143 ***