Dear friends: Problem Solved! I had to donwload a new copy of GAG, install a new copy on a floppy, then reboot, switch to diskette in the Bios and only then does the GAG install/uninstall menu show up. Clicked on Uninstall and GAG not only uninstalled itself from the hard drive but also restored the old MBR. Now Centos boots up correctly. Thank God. Benjamin