How to re-install GRUB into MBR easily?

[Henry Leparskas]

We had some sort of problem in an FC3 installation such that GRUB hangs in 
trying to load Stage1.

Suspecting an error in the MBR, we booted again from the FC3 CD1 and asked 
for 'Update'.
This took us through the dialogue in which the installer said that it 
didn't recognize the boot loader and would we like to install GRUB. We 
answered yes, and that we wanted it in the MBR.

Then, in some insane fit, the installation script tells us that because we 
have nothing to update it is going to exit without reinstalling the boot 
loader.

I recall inflexible behaviour like this from older releases of RH.

How is one supposed to be able to reinstall GRUB into the MBR without 
redoing the entire FC3 installation.

Thanks