Desktop Boot and Lockup

Hi. To generalise what has been happening: I was given one of these computers for free by my school that had been said to work. I opened it up and installed some of the correct RAM (3 GB). I also checked that all the cables were correctly plugged in and they were. The problem I get is when I turn the computer on, this is what happens:

1. The green power on LED indicator turns ON
2. The green hard drive LED indicator turns ON for an extremely short duration (<0.5 sec)
3. The CPU and PSU fans come on at normal speed
4. The hard drive makes a clicking sound and spins up normally
5. It then hangs. Nothing happens, the hard drive LED stays off

The monitor does not detect a signal and turns off with the message "No Signal"

So I tested around moving components and trying the procedures said on the troubleshooting guide.

1. Removed the RAM and resat them in different orders, all correctly in with a click. No result.
2. Changed hard drive twice with two others that had an operating system on them. No result.
3. Used 3 different cables for power supply and monitor VGA. No result.
4. Tried resetting the BIOS using F10 procedure. No result.
5. Reset the CMOS. Not sure if that helped or not.

I am stuck for ideas. The computer has not been started up with any other perephials apart from a keyboard and mouse (which both work in other computers). This computer uses the default CPU which is sat properly. It does heat up and so does the PSU, but not to an alarming temperature. 

Thanks for the help.