System loops on boot

[georgepag]

    I'm having various problems with my PC (Compaq 7PL290 Pressario running XP Pro) but I think it boils down to a Windows problem or a corrupted disk.  I need help on figuring out what to do.  Here's the history (it's a little long but I've been playing with this for a while).
     The system started to hang when XP started to boot.  I found that if, during boot, I did not touch the mouse or the keyboard until the boot process completed the hang did not occur.  The system would still hang at some point, usually after several hours, but the boot would complete.
    Then the system stopped booting at all.  The drive lights would light but nothing happended - nothing on the monitor, no disk activity at all.  I could not get into the BIOS or do a safe boot.  It was like the power was on but the system was dead.  I pulled the memory and tried to boot, no change, I disconnected the two hard drives (system disk and a user disk) - no change, I tried booting from a  different drive and from the XP disk, no change.  I figured I had a blown motherboard but I put the memory back in, connected the original two drives  and tried one more time.  The system started to boot!  The Compaq logo comes up and then the windows startup screen but after a few seconds it now loops back to the beginning of the boot process, compaq logo comes up, windows startup screen and then it starts over again.   I still can't bring up the BIOS screen or get a safe boot.
   One interesting thing is that I disconnected what I thought was the system disk and left the user drive connected and tried to boot. (They are not labeled and I wanted to make sure which was which before I wipe out the wrong drive.)  The system failed to boot because it could not find the windows files.  When I disconnected the user drive and reconnected the system disk and booted, I got the 'windows did not start correctly last time, do you want to do a safe boot' screen.  I selected safe boot but the system just went into the same boot loop.
 Any suggestions or ideas on what to do next.  I'm assuming the operating system is hosed and I need to re-install it but not being able to get into the BIOS makes me think that something else is gong on too.

[pcdoc4christ]

The fact that you are able to boot the computer sometimes and not others points to a hardware failure as the most likely culprit, but a virus might also be to blame.  Do you hear a whirring sound or a metallic ping?  If so, your assumption that the hard drive is going bad is likely correct.  If not, the power supply is probably dying hard or overtaxed, and the symptoms might continue until it is replaced.

[georgepag]

Everything sounds normal.

[pcdoc4christ]

Update the antivirus definitions in your antivirus program and do a full system scan.

Make sure your electrical outlet is supplying the correct voltage (use a multimeter to test it if you know what you are doing), or bring the computer to a repair shop and have them test the power supply or replace it.  

There might be a short on the motherboard, or a problem with one of the expansion cards, so you can try removing all but the necessary the expansion cards to see if the problem ceases.  However, if the problem still persists, you should rule out the power supply as the cause by installing a known good one.