AAAARGH! PC problems

[Blunt]

You explained you fitted a new PSU? what did you swap in it?

I swapped in my old PSU (FSP FX700-GLN) which seemed to fix it at first but it didn't make much difference in the long run.

Sounds like a BIOS setting was stopping Windows from booting, almost certainly a SATA setting.

After you install Windows you can't change the SATA operation mode in the BIOS (eg, IDE to AHCI), doing so produces a reboot loop as you describe. Flashing the BIOS might change this setting.

I think it needs a flash of the BIOS, but I'm going to wait a few days to make sure the system is stable.

[kregoron]

You old PSU might not be able to handle the strain for the system.
because as soon as you start to test the system it crashes, and stressing the system (benchmarks, tests, games and so on) increases the demand for power and if its not capable of delivering.
 
But it could still be a number of other reasons.
 
The reason for your endless loop might just be a broken install so a fresh install might solve it.

[Blunt]

Well, my PC woes have returned:(
This morning I was using the PC and all was ok. I came back to it after an hour or so and it had shut down/off.
I turned it back on and the fans fire up for a few seconds then it shuts down and after a few more seconds it tries to start up again, ad infinitum.
I'm going to try another PSU later, but I fear it could be worse than that.

[Blunt]

I took my PC to a local reputable PC repair company and they sorted it.
It turns out that my pc doesn't like to hibernate, or rather, It loves to hibernate...
so much that it won't come out.
The PC repair man Bypassed hiberfil, booted normally and disabled hibernate and took it off the start menu.
Nearly £50 poorer, but :D:D

[DuVeL]

Shoot you tonight then Pete!

[BrotherTobious]

Glad your going to be back mate