As an update..
Downloaded and ran Spybot....
Downloaded Registry Mechanic,
First run of Spybot revealed absolute mass of problems.... read tutorial and dealt with problems. Big improvement with performance, but still had occasional" Not responding"
Registry Mechanic identified 450 problems in various areas. Once those problems were fixed, ran PC and so far no problems at all.
So not sure what actual problem was but I think it's cured ( for now!)
