Supplanting RHEL with Ubuntu

[petyr_ub]

I have a DELL Optiplex GX 150 with limited memory on which I wish to run EMC2. I installed RedHat (RHEL 5.2) and then tried installing Ubuntu 8.04 over RHEL. It appeared to work (I got to the final bird image) but on Reboot it came up in RHEL. How can I kill RHEL and only install Ubuntu.

Thanks much for your advice