Question : Problem: Looking for info why boot-up fails sometimes

I recently received a monitor message saying "No Signal Detected", whilst booting up the system after being turned off for around 10 hours.  The system is around 4 months old and the montior around 17 months old.  It is important to know that the problem occurred before and after the following checks i carried out in order to diagnose the problem and that the system is now working again even though i apparently have not found the solution.  When booting up the fans are heard to be whirring but nothing seems to be sent from the system back to the monitor.  I have checked the monitor to system cable and all seems ok.  The memory has been checked and re-seated.  I did try my girlfriends JeanTech 700w PSU in my system and it worked immediately.  No components appear to be overheating.  
Listed below are my system specs:
Intel® Core"2 Duo E8400 (2 X 3.00GHz) 1333MHz FSB/6MB L2 Cache
8GB CORSAIR XMS2 800MHz
ASUS® P5K MB
64 BIT WINDOWS® VISTA Home Premium
512MB GEFORCE 8800GTS
700W Quiet Quad Rail PSU

Thanks in advance for anyone taking time to read this question and for any possible solutions given.

Answer : Problem: Looking for info why boot-up fails sometimes

Back to the PSU then, or motherboard.
By the way the voltages look ok but if those were taken when in BIOS then thre may be a difference when the computer is under load.
You can use this 12v PSU calculator to see if your PSU has the needed 12v amps required for your setup.
http://web.aanet.com.au/SnooP/psucalc.php
Can you give us the spec on the sticker on the side of the PSU. It will show the different voltage rails and the amps per rail.
Looks like this.

EPSPOW850PRO-specs-sticker.jpg (32 KB) (File Type Details) 

PSU spec sticker