My computer is about 18 months old. Never had a problem before. Starting yesterday, I can no longer start up my computer. It is hung up in a continually loop -- always!. After 2 or 3 seconds of the normal sounds and lights of a start up, you hear a loud "click" and it automatically starts over and trys to start up again -- repeat, repeat, repeat...... All the various lights on my CD Drive, DVD Drive and green & yellow dashboard lights are flashing as well. The only way to make it stop is to turn off the main power. It doesn't even get far enough in the start up routine to try the F8 save mode. It doesn't get that far and restarts. I've never seen anything like it and I consider myself pretty knowledgeable about computers. Any help would be appreciated.

Note: my first hint that something was wrong happen at shut down. The same basic thing happened. I would click on "Shut Down" and the computer would start to shut off and then suddenly reboot all by itself (no, I didn't select restart!) The only way to turn it off was to manually power it off. Now, I have the problem mentioned above.

More notes: I've loaded XP Service Pack 2. No recent upgrades or software installs. I've tried a bootable floopy and a Rescue CD, but the startup process doesn't get far enough along for it to work.

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Go to your profile and fill in your Hardware specs,It helps to get faster and more precise answers.
Can you get to system bios ? you could reset CMOS
CLean and reseat Vid crd ,Ram.confirm cables are well seated.
You need to get to at least boot to Floppy. If you can then get to Safe mode
Go > My Computer >Properties>Advanced>Settings>Start up & Recover; and clear the check mark for Auto restart under System Start up .That will allow you to get some error messages on screen. You should be able to troublshoot from there.
Get back to us and tell us how far you can get.

I would love to try all the things you are suggesting, however, the startup loop (loop is what I call it) is only a few seconds long. I can't get to safe mode. My bootable floppy & rescue CD are no good since it doesn't get that far in the startup process. Whirl . . . click . . . repeat every couple of seconds.

The first part of my answer is not the Windows boot process it is the Bios ( press DEL or F2 etc.) when you first turn on the machine. And the re seat and confirm connections , I meant is inside the Computer
Poertner and I are both leaning towards some type of Hardware issue.
So you must get the machine stable in order to make changes in the OS.