Just to see if I understand: Mandriva will not boot because the fstab file is corrupted? Or is it not listed in the grub menu?
You can make a new fstab, but be careful...be sure to backup the original, and use it as a reference. The fstab file is specific to the linux installation. In other words, if you install Mandriva, Ubuntu, and Mepis, for instance, they will each have their own fstab, which will tell that installation what drives are available to mount. If one of the fstab files got corrupted, you should be able to boot into a working installation, then chroot into the non-working one to create a new fstab, or even just copy a suitable fstab file to the non-working one (again, just be sure you back up your fstabs, and make sure to copy to the correct partition). If you don't have a working install, you should be able to use a rescue or live cd to fix your fstab.
Whichever install is the latest will overwrite your grub menu, unless you specifically tell it not to. You may have to fix your grub menu.lst file to include all of your boot options.