This error is produced when grub cannot recognize the file system it's second stage files are in. This usually happens when you install grub in the wrong partition, like the Windows partition for example (/dev/hda1) instead of installing it to /dev/hda which is the MBR of the disk.
If this is the case, even after repairing grub, you may not be able to boot Windows because it's boot files have been overwritten by grub.
Put the 1st Mandriva CD or DVD in the drive and boot the computer. Make your way to a root terminal and issue these commands to repair it:
Mandriva also gives the option to repair it automatically, the instructions below are for Mandrake, but Mandriva still uses the same installer so it'll probably be more or less the same steps involved if you want to try this first, which I recommend you should:
1: Boot the computer with the first install CD or DVD in the drive.
2: When given the option to press enter to install or press a F(x)
combination where (x) represents a number for options, press the
appropriate F(x) key to see the options. Most often you need to
type: linux rescue at the prompt if there is no rescue related
3: The installer will load drivers into memory and should present
a list of rescue operations. If a progress bar shows up and goes to
the end and appears to hang there, hit the "Esc" key on the keyboard
to view the list of options.
4: If you see an entry like so: "Re-install Boot Loader", highlight
it if needed by using the arrow keys, then hit the "Tab" key on the
keyboard to highlight the "OK" switch and hit enter.
5: Linux will mount the installation and ask to press Enter to
continue, hit "Enter"
6: It will do it's thing and tell you to press Enter to return to
Rescue GUI, hit "Enter".
7: At the GUI with the list of options, use arrow keys to highlight
"Reboot", then hit "Tab" key till "OK" is highlighted and remove the
disc from the drive, hit "Enter" to reboot.
If you did actually install grub in the Windows partition, you'll have to restore Windows boot with it's installation CD, which you may have to do first.