POST is normal or up to 2 hours delayed

I am hoping this is as simple as it seems. A Windows XP computer built about 2 1/2 years ago AMD 2100 with Antec power supply has been operating normally until two or three weeks ago. If the computer is turned off completely (i.e. unplugged from the wall) and then turned back on the next morning, it can turn on normally but it can also take up to two hours for it to suddenly POST and from that point it runs normally. Some drive lights come on and then it will suddenly post normally after a long delay.

Also odd behavior is this: The computer will do an XP software shutdown but remain partially on, especially blue light installed in the case. (This is the reason I have had to completely unplug it at night sometimes).

Failing power supply? Or possibly a more scary failing C drive? Is it clearly one or the other? If not is there a way to distinguish which it is?

Mcafee and Spybot are up to date, always on and detecting nothing...

The C drive data has been backed up to the D drive.

Let me know what you think....