Hi, Jonathan. Many thanks for your assistance.
I'm making my way through your post. I did download and run the CPU test, and after a short while in, I got this:
[Oct 1 23:27] Worker starting
[Oct 1 23:27] Setting affinity to run worker on logical CPU #0
[Oct 1 23:27] Beginning a continuous self-test to check your computer.
[Oct 1 23:27] Please read stress.txt. Choose Test/Stop to end this test.
[Oct 1 23:27] Test 1, 800000 Lucas-Lehmer iterations of M172031 using FFT length 8K.
[Oct 1 23:29] Test 2, 800000 Lucas-Lehmer iterations of M163839 using FFT length 8K.
[Oct 1 23:32] Test 3, 800000 Lucas-Lehmer iterations of M159745 using FFT length 8K.
[Oct 1 23:34] Test 4, 800000 Lucas-Lehmer iterations of M157695 using FFT length 8K.
[Oct 1 23:37] Test 5, 800000 Lucas-Lehmer iterations of M155649 using FFT length 8K.
[Oct 1 23:39] Test 6, 800000 Lucas-Lehmer iterations of M153599 using FFT length 8K.
[Oct 1 23:40] FATAL ERROR: Rounding was 0.5, expected less than 0.4
[Oct 1 23:40] Hardware failure detected, consult stress.txt file.
[Oct 1 23:40] Torture Test completed 5 tests in 13 minutes - 1 errors, 0 warnings.
[Oct 1 23:40] Worker stopped.
Is that anything significant? I will let you know how I get on with the other things you suggest.
In answer to the other couple of questions asked, I've ran chkdsk 3 times now, and all has been okay. My PSU is a 500W (Peak) Quiet Dual Rail PSU + 120mm Case Fan.
Many thanks again.