Cheers for the replies, chaps.
Forgot to mention in my originalpost - The problem only started after I'd downloaded and installed the last MS update.
BOAC,
The virus scan was performed by my AVG prog; first the "Scan Whole Computer" option, followed by the "Rootkit Scan".
Someone on another Forum also suggested Malwarebytes.
I'll give it a whirl and report back.
Rev I. Tin & jcjeant,
I'll check the PSU. Although it is the original unit (PC is around 5 years old), it seems to cope OK with the few online browser games I play, one of which is quite demanding on the system.
Could it be a dust build-up?
Again, I shall check when I have some spare time.
Are there any other options to consider?