Grub error 21 indicates selected disk does not exist so something went wrong with the bootloader installation.
Post more information, number of drives/partitions? which bootloader are you using? How did you get the error, trying to boot Mepis, windows, both?? Did you install Grub to the master boot record? Of the first drive? Do you have multiple drives? The most detailed information you could post would be if you went to the site below and downloaded the bootinfoscript and ran it after reading the instructions. You need to do this from Linux so use the Mepis CD: