You can rename your menu.lst to another name, say rubbish.txt. Grub will then be able to boot to a Grub prompt to enable you to boot the systems manually.
Believe me there is no installed operating system that cannot be booted by a Grub prompt.
In a Grub prompt you can fire up your renamed menu.lst any time with command
If your grub configuration file is healthy then Grub can boot it. Without it Grub defaults to a Grub prompt.
You can fire up your Mandriva manually in the following manner
If /boot is in sda5 and / is in sda6 you tell Grub the root is sda5 or (hd0,4) in Grub's term
You then type
and hit the tap key so that Grub searches the partition to find a file that match the beginning. If it find the kernel it will respond by adding the missing bit which mean you have hit the correct partition and directory. You then add "ro" for ready only and state the location of the / like this
kernel /vmlinuz-2.6.11-8mdk ro root=/dev/sda6
you then add the ramdisk file again let Grub do the first part as it will find the missing bit Iindicated in red
If Grub has no complaint then there is a 85% chance your Mandriva will boot if you give Grub the green light by command
The above is to bypass the menu.lst completely. Once you find the correct instructions then edit the menu.lst so that you don't have to boot it manually every time.
Believe me you don't need Supergrub. Just tell Grub directly what you want! You can get everything the horse's mouth this way.