Big Mac Pro issue (1,1 - 2006)
With a machine that old, I'd probably use a different Mac to download a clean OS installer and then try installing to a totally new drive (USB or FireWire) to ensure you have working hardware that can run an install.

Installs exercise a lot of functionality that only runs at install time. A marginal machine can keep running for a long time past when it can actually pull off an install let alone an install/upgrade in one pass.

The benefit of this is you keep your data safe (assuming you don't have a backup) and don't have to wait hours/days for migration of settings and data to know if the simple OS install worked or not.