Desktop - PSU & HDD trouble

[Puter Moron]

I have been given a desktop that “doesn’t work anymore”.  The computer had been showing a message on a "black screen" to the effect that a file was destroyed or missing.  The computer never made it to the desktop.  What was attempted to fix the problem was to shutdown and restart the computer multiple times.  Then the computer would not start anymore.  There isn’t much info to go on but I do know it was running XP and has an Intel Celeron processor.

I am thinking this machine has couple of problems the first is the PSU.  I checked the voltages at the 20 pin ATX connector and got the following:

+3.3v = 0v
+5v = +1.2
+12v = +0.9v

These voltages seem a little on the weak side. 

The other problem would be the harddrive not booting to the desktop and giving a destroyed or missing file message.  The HDD in question is a WD800JB caviar SE and is connected via IDE cable.  I would like to connect the suspect HDD to my computer (HDD is connected via SATA) to test it but I am not sure how to do this with a SATA and IDE HDD and which tests would need to be run. 

[rthompson80819]

These voltages seem a little on the weak side.

Almost on the nonexistent side.  Try another PSU and see what happens.

[Geek-9pm]

The voltages are not valid if the thing is not running.
You should be able to remove all power from the hard drive, And remove the IDE cable too. Now try the power on button.

At this point you may get a screen.

If not, power down fully. Now remove data and power from both floppy and CD ROM drive. (We are thinking a drive may be bad.)
Put power on again

Notice is the keyboard lights come on.
Expect three lights to come on, then off at power on. Then maybe on more time. And last just the Num lock comes on.

If you get that far, Hit the Caps lock. If you can toggle the Caps lock, the PC is more than 90 per cent good.

So how far do you get with no drives connected?

BTW, just how old is the machine?

[Puter Moron]

Okay I don't get it.  Before we went out of town for Christmas the computer would not power up and now it does with all drives connected and powered.  I do see the screen with the corrupt file now and it is:

Windows could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM

I do not see the lights on the keyboard light up and the machine was new in 2005.