I have a desktop pc, I am trying to fix, it won't boot. I have checked the voltage ariving at the hard drive on an it is 6.5 volts, on my other working pc the voltage is 10.5 volts. Does this mean that the power supply unit is broken? what voltage should it produce?
The way to test a power supply is to test the +5v and +12v rails. You can do this by plugging the leads of your multimeter into a molex connector. see my URL below to get an idea of what I mean 
Negative to black, positive to red = 5v test
Negative to black, positive to yellow = 12v test
If both rails stay around 5v and 12v while the computer is under load, then your power supply is working correctly.
Note: Windows-based voltage monitoring tools are usually not very accurate — if your motherboard allows voltage monitoring at all. The only way to know your actual voltage is to test it physically.
You can use this to check system voltages.
cpuid.com/hwmonitor.php
try to read your motherboard manual its all there that you want to know about the power requirement of you pc