You can install lilo without uninstalling grub (otherwise you will remain with an unbootable machine). Just install lilo with the software manager, edit the lilo.conf file to suite your needs, then run /sbin/lilo to overwrite the Master Boot Record (if you installed grub there). If something goes wrong you can always reinstall grub into the MBR.
Anyway, before proceeding better to do a GRUB boot floppy or a GRUB bootable CDROM. For details, refer to the grub manual
here.
