shutting down when relay clicking noise is heard?

[lectrocrew]

 I'm attempting to get an old HP Pavilion 540n PC running to use for Folding @ Home for our team and I'm having some issues. It boots to Windows XP fine but shuts down after a 'relay clicking' type sound. This happens at different time intervals and seems to not be related to what task the machine is performing. It had a bad IDE optical drive (CD RW) that I replaced with IDE DVD-RW but this had no effect on this issue. I bought a 1Gb PC3200 DDR RAM module to replace the 256Mb DDR module but this didn't help either. I've blown all the tons of dust out of it and re-applied thermal paste to the P4 processor/heat sink. It's was running XP SP1, and I replaced that OS with XP SP3 OEM (which I have correct code and registration for in my name) because Windows update would not update SP1 (support expired). The outputs on the PSU check correct with DVOM, processor temp is within operating range, BIOS settings have been set to default.
 What should I check now?
As always, thanks for any help!!!
Mike 

[jason2074]

You might want to check safemode to check if its a hardware or software issue. If it shutsdown, then its hardware definitely. Next would be a HD test, most manufacturers have diagnostic utilities that you could run in DOS mode. PSU would be another factor so try using or swapping of same or greater output. Do you use a video card or just the onboard?

[patio]

The HDD clik of death sounds similar to a relay clik...
DLoad and run the Free diagnostics from the HDD manuf. site.

[Computer_Commando]

...It had a bad IDE optical drive (CD RW) that I replaced with IDE DVD-RW but this had no effect on this issue...

IDE cable is bad.

[patio]

Or...the HDD.

[lectrocrew]

 My fault -  I installed a 'known-good' IDE hard drive and cable and results were the same.
 I was in a rush to get this machine folding and forgot I had it's power supplied via UPS, which is where the relay clicking sound/failure was coming from. I have it up and running fine now and running F@H v6.23.
 Sorry about not clarifying that in my original post but I actually forgot and I,ve never had an issue with this UPS so... now I'll need to find out what needs to be done with this UPS (Best technologies Patriot 280), if it's worth the repair cost 
Thanks for the help guys!!!!