Ah, showing my ignorance here - I didn't realise that CCleaner did the same as these other programs in terms of repairing the registry rather than just clean the muck out of the machine! I've used CCleaner for some years and am continually amazed at the amount of rubbish it gets rid of each week.
The pop up message that says you have a registry problem that can be corrected by buying some software is quite a common virus type problem and can be difficult to get rid of !
Actually, it wasn't a pop-up, but an e-mail from Winzip advertising Uniblue which looked (and, I think probably was, genuine). It was only my natural curiosity that made me investigate further.
Thanks for the recommendations,
GG.