When I start my Red Hat Enterprise 5.1
It stucks at:
GRUB _

The cursor (underscore) blinks but my keyboards don't work

How to solve that problem?

Use a live cd and check the contents of /boot/grub/menu.lst
Make sure that the relevant versions of initram and vmlinuz are in /boot/
If you can't see anything wrong, you can just reinstall grub:

1. Boot off the 1st RedHat CD
2. At the boot prompt type 'linux rescue'
3. The rescue mode will find the RedHat installation
4. Type 'chroot /mnt/sysimage' (without quotes)
5. Type 'grub-install /dev/sda' (to install Grub to the MBR of the 1st HD) - it might not be 'sda' on your system.
6. Restart and you should get the boot menu

Alternatively, you could use the supergrub disk.

Are you using a USB keyboard?

Is this a new install ?. Do you have a support contract with RH - have you asked them ?.

In order to get a clearer picture of your setup, how about downloading the "boot_info_script.txt" to your desktop, and then do the following as root user:
That will create a "RESULTS.txt" file in the same directory from where the script is run, namely your desktop; please copy/paste the contents of that file to your next post. That will help clarify your setup and hopefully what your problem might be.