BE VERY CAREFUL WHEN USING DD, IT CAN COMPLETELY DESTROY THE DATA ON YOUR DRIVE. DO NOT USE THIS INFORMATION WITHOUT BACKING THINGS UP AND VERIFYING EVERYTHING YOU TYPE.
Open a terminal window and type fdisk -l to find out the drive assignments. In the following command the drive you want to erase is noted as (drive). The MBR is stored in the first 446 cylinders of the drive, so we want to wipe those cylinders. Type:
sudo dd if=/dev/zero of=/dev/(drive)/ bs=446 count=1
Once this is finished you can reinstall Grub, LiLo, or any other boot manager and restart.
If all goes well you'll have fixed your MBR woes.