I don't exactly understand why you want to do it, but I'm sure you've got your reasons. I'll just repeat it once again:
If you bought Red Hat support, they can guide you through each step. If you haven't bought it, RHEL will be a bit useless as
you won't get any updates. In that case, it's so much better to use CentOS or Scientific Linux.
If you've got 20GB of free space, you can just boot the computer from the RHEL5 cd/dvd and start the installation process.
Make sure you use unallocated space for your new installation. For installation instructions see, RHEL website: