First, it sounds like you need a new CMOS battery. They aren't expensive and pretty easy to replace. If you replace the battery and continue to get those errors, I would suspect a bad board, after checking a few other things.Try booting with only 1 stick of RAM. Try it with a different stick of RAM, but still just 1 at a time.http://www.memtest86.com
has a good memory tester that you can burn to a CD.
It isn't that odd for the computer to come on for a second, when you turn on the UPS. We sell HPs and they tend to go on as soon as they are plugged in.
You should try it with a different power supply as well. It would never hurt.
If you still cannot boot from the first SATA plug, I would call the motherboard a frisbee and toss it way up.....