Boot up from any Linux Live CD, open a terminal and do this:
fdisk -l ##this tells us what all your drives and partitions are. find the one containing the Linux OS**---let's assume it is sda2.
mount /dev/sda2 /mnt/linux
sed '/^#/d' menu.lst ##Post the output of this here
**specifically, we need the partition containing /boot. If there is only one linux partition, then that's it. Otherwise, when you mount the partition, look to see if /boot is there. If not, try a different one. (If there are 2 Linux partitions, /boot is almost certainly on the smaller one.)