a bit late , but ...

also Please PAY red hat for the updates ( buy a license and access to RHN )
RHEL5.5 is NOT FREE

you problem is grub, use a live distribution

The first thing to do is start the live-cd and open a terminal. Then write the following to see the partitions on the drives:

$ Sudo fdisk-l
Then we see which is the partition where Ubuntu and we got on / mnt (in most cases, this partition is sda1, the example I will make that partition but look what your partition with the fdisk command):

$ Sudo mount / dev/sda1 / mnt
Now mount the other devices:

$ Sudo mount - bind / dev / mnt / dev
And running the chroot command as root so that we access the file system of our old Ubuntu:

$ Sudo chroot / mnt
Finally load the Grub in the MBR by running the following command:

# Grub-install - Recheck / dev / sda
(Sda it should be replaced with the hard disk used to boot operating systems, almost always sda. Ojo! Not put the number of partition, only sda)

Reboot and when to restart ubuntu (not the live-cd), we can adjust manually grub menu to display the boot menu in the new operating system we deleted the MBR, or let him do it automatically the following command:

$ Sudo update-grub2
If the command does not work, you may need to install the package grub2:

$ Sudo aptitude install grub2

Note. If you have a boot on separate partition
After mounting the file system on / mnt (with the command sudo mount / dev/sda1 / mnt), you must also mount the boot partition with the following command:

sudo mount / dev/sda2 / mnt / boot

Note the commands are examples, you put the partition where you installed linux

example here
hd0: the first full disk, like hda or sda
hd0, 0: is the first partition of first hard disk, like hda1 or sda1
hd0, 1: is the second partition of first hard disk, like hda2 or sda2
hd1, 2 is the third partition of second hard drive, like hdb3 or sdb3