G5 blue screen = flat battery
Long story cut short: Original model G5 Powermac was showing a blue screen at start up, and wouldn't get as far as the log-in screen. It turned out to be a flat PRAM battery.
More detail: Drove me nutty for over a week! The machine would not boot the regular OS, but would boot OK in Safe Boot mode (holding the shift key). This would normally indicate it was a software problem, but not so. Eventually, the machine didn't even want to shut down without being forced.
Apple has many support pages about how to deal with a 'blue screen' failed startup. The listed solutions include various software remedies, including removal of Startup items. It does not mention the flat battery issue. I spent long hours attempting software remedies which all failed. After purchasing and installing a new half size AA 3.6v Lithium battery from the local electronics store, the machine is working fine again.
The original G5 Powermacs are approaching the age when their internal batteries will be going flat and need replacement. If anyone from Apple is reading this, it may be worthwhile to create a specific support page for G5 Powermacs and Blue Screens.
I post this message for all those wonderful people who own G5 Powermacs who may have blue screens or strange start-up issues, who will Google for an answer and find this post. I wish you well with your G5 Powermacs!!!
G5 2x2GHz, Mac OS X (10.4.11), 2.5 gigs of RAM