Can't boot

I'm new to this forum and hope that someone can help. I'm fairly computer savvy and do most maintenance and repair on our small office computers. Both are HP P7-1202 with 6GB memory and a Seagate 7200.12 1TB hard drive. My problem is that one of the systems won't boot. It has been acting funny (stopping, sometimes screen flashing blank) for a few days. I thought the hard drive was failing, but I was able to get it running yesterday and bought an external drive that I was going to mirror using the Carbonite software to do so. It failed again, however, and I can't get it to boot again. The hard drive sounds like it is spinning up and the drive indicator light flashes for a few seconds, then nothing.

I swapped the HD with the one in the other system and it booted up just fine. No HD problem, I guess. I had also run some HD diagnostics yesterday while it was running, and no problems were detected. I then swapped memory modules and again no problems. No matter which drive or memory modules I put in the "bad" system, it will not boot.

Could this be a power supply or motherboard problem? I've never encountered one of those problems in the many years I've been working with PCs. I haven't swapped power supplies yet, but plan to.

Any suggestions?

Update: I just checked all the power supply connections with a DVM (while they were plugged in - connectors back probed). All spot on. I guess that leaves the motherboard, unless someone has another suggestion.

Another update: I'm beginning to suspect a video failure on the motherboard. It has integrated video. There are two video outputs and it makes no difference which on I connect to. Still nothing.

Does the PC still perform POST? Do you seethe POST screen? If not, better check the capacitors on your board. Check if there's anything bloated or leaking. If you do see POST, I'm with @whs on this one, try a cheap compatible video card.

Actually instead of buying a video card. Why not spend 5 bucks or so for a USB/Parallel Laptop Motherboard Analyzer/Diagnostic Test POST Card 4-Digit Codes. You can determine exactly what is wrong with your system without having to spend a lot of money guessing on internal parts. It works with laptops and PCs. I use it all the time in my business. Saves me time and money.

Kenna said:

Actually instead of buying a video card. Why not spend 5 bucks or so for a USB/Parallel Laptop Motherboard Analyzer/Diagnostic Test POST Card 4-Digit Codes. You can determine exactly what is wrong with your system without having to spend a lot of money guessing on internal parts. It works with laptops and PCs. I use it all the time in my business. Saves me time and money.

WOW. I seriously need to get me one of these! Can you guys suggest a good brand and model? I gotta look it up now.

Here's the latest. I wanted to make sure the monitor and cable were really ok, so I connected them to the "good" system and they worked just fine. I moved them back to the "bad" system, and guess what, it worked! Although I've never encountered one, I'm begining to suspect the cable or a bad connection - although I checked them before for tightness. After I get all my backups up date and my HD mirror image made, I'm going to do the "wiggle" test of the cable. Apparently, all the times I rebooted, it must have been working fine (got no POST beeps) - just no video to the monitor. It kept saying "no input" and "going to sleep." I'll post again after the wiggle test. BTW I did inspect the MOBO for bulging caps. Didn't see anything suspiscious. I was considering a video card, but I thought if it was a MOBO problem, who knows what might have happened next. I did find a new MOBO just like mine for $100 w/shipping. Seems a fair price, but never having bought one before, I don't know. Thank you everyone for the help.

To Synth,
Yes they are not that expensive and can save you hundreds of dollars of expensive trial and errors in troubleshooting.
You can purchase on for use with a laptop and PC here:
http://dx.com/p/usb-parallel-laptop-motherboard-analyzer-diagnostic-test-post-card-4-digit-codes-33304
USB/Parallel Laptop Motherboard Analyzer/Diagnostic Test POST Card (4-Digit Codes) Diagnose the motherboard signal which shows the problem of CPU, Memory, and display card and so on

I recommended it for Ramjet1121 to help prevent him from spending too much money on parts without knowing what is wrong with the system.