On the system I was working with, I've pretty much narrowed it down to the power switch. I had to upgrade the RAM in the system, and when I plugged it back in, the machine started up immediately. I went into the BIOS, and made sure that the system was set to be powered off after power was restored (it was), and all other power management settings seemed OK.
Hibernation is not a required feature by any means (most useful on laptops, rather than desktops). All it does is save contents of RAM to your hard drive before powering off the computer. Of course, with systems now having 2-4GB+ of RAM (mine has 8GB), it takes longer to resume from hibernation than it does a cold boot.
That said, on a completely off-topic note, Windows XP, Norton Antivirus, and 256MB of RAM is a surefire way to kill a system.... horrible stuff.
