I tried uninstalling IE8, reverting to IE7, but the problem continued there (and have now re-installed IE8).
This problem appears to have spread to other strange mouse click responses e.g. in MS Outlook a single left click on an email sometimes opens the mail in a new window whereas before it just became visible in the reviewing pane; e.g. clicking the card deck in the Solitaire game (that came bundled with Windows) sometimes turns cards over twice.
I have run the following, which have reported no problems found: MS Security Essentials full scan, MS Onecare full service safety scan.
If anyone can offer any help with trying to resolve this I'd appreciate it.
Doh! Forgot to try the most obvious - see if a different mouse (or the touch pad) has the same problem - which they don't, so guess this was a hardware problem after all. Sorry guys, didn't realise a mouse could develop a fault that would exhibit these symptoms (and the symptoms boil down to a single left click being translated into a double).
Edited by t-s, 25 December 2010 - 02:01 AM.