Did something to fstab and now can't boot the machine? Easiest thing to do is start up a Live CD and mount the Linux partition with the fstab that needs fixing on it.
But Live CD can't mount it?
this will (just find out the disk name)
sudo mkdir /media/sda1
sudo mount -t ext3 /dev/sda1 /media/sda1
sudo leafpad /media/sda1/etc/fstab