Ubuntu 9.04 uses a newer kernel which renders older graphics drivers
obsolete. It isn't 9.04 that is the problem but the kernel 2.6.28. You can
install an older kernel and remove 2.6.28 and use the old graphics drivers.
This would be easier than re-installing Ubuntu.

Look in the repositories in Synaptic for kernel 2.6.27 and install that. Do
not remove 2.6.28 until you boot into 2.6.27 successfully. Make sure that
the kernel matches your current one in all things but the number. It will
most likely be called 2.6.28.12-generic. You want the most recent one that
has 2.6.27.x-generic.