Try this:
example:

title Knoppix
root (hd1,0)
kernel /boot/vmlinuz-2.6.6 root=/dev/hdb1 ro ramdisk_size=100000 lang=us apm=power-off hda=scsi hdb=scsi hdc=scsi hdd=scsi hde=scsi hdf=scsi hdg=scsi hdh=scsi nomce vga=791
initrd=/boot/initrd.img-2.6.6

** change /boot/vmlinuz-2.6.6 and initrd=/boot/initrd.img-2.6.6 to the kernel image you want to use(check in /boot).