error loading os
i'd almost be willing to wager you erased your master boot record (MBR) when you tried to install GRUB. which was kinda the point, so don't think you did anything wrong. did you want MEPIS to do a clean install (that is, wipe everything from the hard drive and install) or did you want to dual boot? i assume if you wanted it to format the hard drive, you weren't looking to keep whatever was on the hard drive...
mepis didn't install, so try and install it again. if you still get the 'error loading operating system' message, re-download mepis, and try it all over again. and if that doesn't work, try another distro. and if that doesn't work, your hard drive might be messed up, to where the mbr-section won't be written-to anymore. which would mean you'd need a new hard drive.