HOWTO: Restore GRUB (if your MBR is messed up)
5,196 views1. Boot from a Live CD, like Ubuntu Live, Knoppix, Mepis, or similar.
2. Open a Terminal. Go SuperUser (that is, type "su"). Enter root passwords as necessary.
3. Type "grub" which makes a GRUB prompt appear.
4. Type "find /boot/grub/stage1". You'll get a response like "(hd0)" or in my case "(hd0,3)". Use whatever your computer spits out for the following lines.
5. Type "root (hd0,3)".
6. Type "setup (hd0,3)". This is key. Other instructions say to use "(hd0)", and that's fine if you want to write GRUB to the MBR. If you want to write it to your linux root partition, then you want the number after the comma, such as "(hd0,3)".
7. Type "quit".
8. Restart the system. Remove the bootable CD.
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http://ubuntuforums.org/archive/index.php/t-24113.html