Turning on, and off sounds like bad capacitors on the motherboard.
I don't believe the members of the organization you took it to; have the savvy to repair.
(Frankly I don't think they could pour P out of a boot..................Wait!...............Did I say that out loud?)
This is what it states from Microsoft for 'Unmountable boot volume',http://support.microsoft.com/kb/555302
The jist is the Harddrive cannot be reached - Bad IDE ribbon cable
The Windows files are corrupted - Use Restoration Disk
BIOS Settings are set wrong.
I'm going to state something different. May be putting my neck out...
Capacitors on the motherboard.
To be specific the ones used on your motherboard are Electrolytic Capacitors.
More specifically; Radial Aluminum Electrolytic Capacitorshttp://h10025.www1.hp.com/ewfrf/wc/document?docname=c00363478&tmp_task=prodinfoCategory&cc=us&dlc=en&lang=en&lc=en&product=457247#N394
An Electrolytic Capacitor has Electrolytic paste inside.
When the capacitor starts to fail, it's because the paste has developed a gas inside. Hydrogen Gas. The gas expands, and pops the seals of the capacitor open.
So much paste loss, and the capacitor operates at a weakened state.
Too much paste loss, and the capacitor fails.
Capacitors are used as Voltage Regulators, and Filters.
The ones used as Voltage Regulators, are in the Motherboard Voltage Regulator Circuit,http://www.hardwaresecrets.com/article/616
Part of what the motherboard voltage regulator circuit does; is to regulate voltage for the Processor.
The Processor MUST have a steady, 'clean', supply of voltage; and it MUST be kept within the small tolerance range, for the Processor.
Cannot be too much, or too little; or BIOS will turn the Processor off.
The Processor is the 'Brain' of the computer.
Guess what happens if it turns off, turns on, turns off, etc?
I think what you are seeing.
Cannot access boot files on the harddrive.
Computer off, and unplugged from power.
FOLLOW Anti-Static Precautions.
Your body carries Static electricity. Static WILL fry out (Short Circuit), the delicate hardware components inside a computer.
Relieve your body of Static BEFORE reaching inside your computer.
Computer ON a table, computer unplugged from power, computer case open;
TOUCH an unpainted surface, of the metal frame of the open computer case.
This action will relieve your body of Static.
IF you leave your computer in the middle of working on it, be SURE to Touch the metal frame again, upon your return.
There are going to be cables (Wires), in the way. You can gently move them aside to look at the capacitors. L@@K close. May need a bright light, and magnification.
Capacitors look good, I'm full of beans?
OK. Onward through the fog.
Replace the IDE ribbon cable going TO the harddrive.
Test the Power Supply, or see if you can borrow a KNOWN to be good, Compatible power supply; from a working computer for a test.
I think capacitors, therefore motherboard replacement.
Not advertising for seller, nor website. Just from a quick search,http://www.ebay.com/itm/ASUS-P4GV-LA-Micro-ATX-Motherboard-Intel-SL7TS-2-93GHz-CPU/130808856405?rd=1&ssPageName=mem_guide:3&ih=003&category=131511&cmd=ViewItem
MAKE notes, and drawing/s as to where EACH cable plugs into.
The ones I may not be able to help you with, are USB, Front Panel audio, Front Panel Firewire (1394a)
Power cables, data cables to optical drive/harddrive, plus Front Panel header; yes.
For additional questions please post in a Comment.