Two tone alarm signal as the screen goes blank - what's wrong?

System: VIA Technologies Inc KT400-8235
Motherboard: Biostar M7VIT Pro
Processor: AMD Athlon XP 2500+ 1.8 Ghz
OS: Windows XP SP2

Two days ago my computer powered up normally until reaching the desktop whereupon it powered off abruptly and a strange two tone beep alarm sounded from the MoBo (like an ambulance siren).  It continued for a few seconds even after I pulled the plug.    After a pause I tried to boot up again but got the warning tones right away.

What is this?  ......  PS failure,  Fan failure, CPU or Motherboard problem?

With the case open I notice that when I hit the power switch the CPU fan twitches but does not run.  I removed the fan and cleaned it as well as removing a lot of dust and fluff from the heatsink.  I tried a new fan with the same result.    Now I suspect the Power Supply but I am wondering if it is time to replace this 3 and a half year old system  -  would rather not if the problem isn't too expensive but I don't want to spend $90 - $120 on a new power supply if the cpu or mobo are fried
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enquirerAuthor Commented:
I wanted to take a look at the processor but I have been defeated by the clip holding down the heatsink. I haven't a clue how to undo it.  The processor is AMD Athlon XP 2500+  1.8Ghz
try to disconnect as much devices you can, leave only :
mobo + cpu + 1 ram stick, video card  -->  still siren? then swap ram or video card.
If still problems, it is mobo or cpu.
if no problems, add devices 1 by 1
It almost sounds like an overheating alarm.  This particular processor has earned a reputation as a strong overclocking processor.

1. Check the cpu temperature with a temperature probe.
2. Check your fans to make sure you are getting enough cooling.
3. Adjust your overclocking (take it back to factory normal)

Remember, if you are overclocking (which I suspect you are with this cpu), the faster you make the cpu run the hotter it gets.
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enquirerAuthor Commented:
Thanks both responders.    I have not ever overclocked the cpu - wouldn't know how to do that -  unless it can occur without my interference merely by running a lot of stuff at once.

Thomaswright,  I can't check the cpu running temperature because the pc is totally cold and dead. I get the siren noise within one second of hitting the power button, accompanied by the fan 'twitching' briefly and stopping.   Initially though I did think of overheating right away.  According to the totally inadequate manual for the mobo the cpu has 'over temperature protection'.

Nobus,  I will try your recommendations and see what I can learn. At the moment the power supply is removed from the computer and I was going to try to have it tested as that is my new suspect  ....  I am working in order of cheapest solution first .......  Assuming there was a problem with the cpu or mobo would that stop the fan from running?  One would think that if the cpu was overheating, its 'protection' feature would make sure to keep the fan running.    Hence .......  PS problem?

Have to go to work  .........  will check back later.
The alarm code you are hearing should normally be short and long beeps. If it really does just sound like a "warbling" ambulance type tone, that normally means overheating. But if the PC won't start up at all, it's unlikely it's overheating, since it won't even turn on. (To test, try leaving it off for a few hours, then turn it on.. if it still won't turn on, then I'd say the first thing to check is the power supply)

You can pick up a cheap 350 watt power supply that should be fine for an older PC like yours for around 30 dollars. You could find a place with a no restocking fee return policy (like fry's electronics) and then try it and see if that fixes it. Power supplies dying after a few years is very commonplace.

As far as the age of the computer, it is definitely old and it would be a good time to replace it, BUT a 2500+ isn't THAT bad either, depending on what you're using it for. For cutting edge gaming, it would be a disappointment, but for web browsing and older games and such, it would be just fine.

If you've been thinking about upgrading anyways and you have the money, new computers are fun. If you'd like to save your money, I'd try a new power supply first...

If that doesn't do it, then it's likely the motherboard or cpu, at which point you'd probably have to do a new PC anyway just about.

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enquirerAuthor Commented:
AndrewCink   -   You got it!

Your comment encouraged me to rip out the psu and I took it a local computer repair shop where they were able to test it right away.  It was dead  - no power output at all.  They were also able to sell me an inexpensive new power supply for about $30 (Cdn).   I installed it with some trepidation, very carefully and replaced the old fan and put the RAM back in.   I kept my fingers crossed as I hit the power button and was delighted to see the fan spring to life and then hear that single, most welcome beep from the mobo!  I've had the system running for several hours now and it is operating flawlessly and quietly  -  and it could be my imagination but it seems to be faster generally.   This was definitely the scariest crash I have ever gone through with a computer and I felt for a while that it was quite likely toast.  It was also probably the most satisfying fix I have ever done.    I think your advice was spot on  and so all the points are yours.   Thanks muchly!
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