Touch wood I am now back to normal on the computing front. I’m now operating on a spanking new PC, and I also treated myself to a tablet PC for travel and fallback (largely on Tony Goodson’s enthusiastic reviews)
As for the old machine, I concluded in the end that is was a hard drive failure due to excessive use rather than a virus. Fortunately, I got all my crucial data off it except one Digital Certificate, so things could be worse.
As for buying an Apple… well, maybe next time!