Won't boot, stuck in Logo with spinning dots

[colmde]

I have a HP Pavillion with Windows 8 which is refusing to boot.

The Windows logo (this was the HP logo for a while but during my attempts to fix it changed to Windows logo) appears with the spinning dots at the bottom but it will go no further.

Sometimes I get "Preparing Startup Repair" on this screen but after a few seconds the screen goes black and stays black.

What I've tried:

- In the diagnostics / System test (i.e. pressing F2 on startup): Quick and Extensive test. Both report all systems as "Passed".

- Boot from a HDD from another computer and displays boot up messages: Eventually it gets to where I think the Windows logo would be displayed but the screen goes black and nothing further happens. I've also tried with this HDD in "Safe Mode". But then it just gets to the end of the boot up messages and freezes.

- Boot from a USB key with "Gandalf's Win8.1U1SE_x64 updatable final" on it: This shows the "Loading files" screen. But when that's complete it goes back to the Windows logo with the spinning dots and gets stuck there as before.

As per this question 

- Plug out device and hold power button for 15 seconds ("Hard reset"): When I reboot afterwards it's the same problem but now the Windows logo appears to have changed its aspect ratio slightly!

- Reset the BIOS: BIOS reset works fine but still have the same problem. Also, now the BIOS won't recognise my USB key for some reason!

- Press F11 on startup to force system repair per this question  : It displays "Preparing Startup Repair" but as above the screen then goes black and stays black. The fan runs very loud for a few seconds.

I'm really at a loss for what to do next, also how do I get my USB key back in the BIOS? 

BIOS is v2.15.1234

Just another piece of info - I can access the HDD from another PC. But if I try to boot directly from this Disk (rather than the Windows Boot Manager) it tells me it's not a boot disk. 

[itsmyname]

What happens if you boot the UBUNTU variant of LINUX from DVD (or USB) ?

Does it boot to a point where it asks whether to install UBUNTU or to run in "LIVE CD" mode (loading UBUNTU into RAM, but not writing anything to your disk-drive)?  If not, this implies a hardware problem.

Or, can you connect the disk-drive as a "secondary" disk-drive in another computer, and run "CHKDSK" against this drive?