No need to backup /boot.
Go ahead and load SP1 for Vista and enjoy the enhancements. When your boot loader gets restored to the default Widows boot loader, boot with your Fedora disk (or practically any Linux disk) and enter into rescue mode. Once you are there, let the restore environment automatically mount your Linux distribution.
Your Linux installation will be mounted to /mnt/sysimage. Chroot into /mnt/sysimage and issue grub install /dev/sda
. After a receiving a successful response, reboot.
Since you already had a dual boot configuration, there is nothing more to do. Grub will (should) easily read your grub.conf settings.
NOTE: If you can’t see the restore mode when you boot with a recent CD or DVD release of Fedora, hit the F2 key to see the various boot options. In there, you will see instructions to type “linux rescue” to enable rescue mode.
As always, good luck.