I hope you're paying redhat for using RHEL 5.1 and availing their services. Right? If so why don't you take their support, they provide very good support, either you can call them or you can login into your account and create a new case.
Well, you can try this but I'm not sure.
At the time of installation use this boot option boot: linux text pci=nomsi
As I told you I'm not sure, so you can try this at your own risk.