From MEPIS Documentation Wiki
Rarely needed, most of the time the problem can be fixed without a reinstallation. That being said, reinstalling MEPIS is extremly easy (it might take less than 15 minutes):
- Boot the MEPIS CD.
- Select the kernel and resolution that work best for you.
- Log in as root (password is "root").
- Run installation center (use the same settings as you did first time, same partitions, same file systems).
- Don't forget to select the option to preserve the /home partition or folder.
- Use the same username and password.
- Respond "yes" to the question if you want to use the old user account
- Reinstall GRUB in the same place you put it when you installed MEPIS first time.