Everything failed - reduced use of machine to holding email archive and local back-ups before backing up to offsite and confined whole issue to the 'too difficult file'.
Pending imminent visit of grandaughter (who was permitted to use machine under her own limited access user name only) who has winged about non-existance of "her computer" and kept scrounging one of our individual machine, I threw all work out yesterday and have spent last 36 hours trying to solve.
With the aid of google and mucho patience - identified a corrupt USB driver - disabled it and heh presto - machine flies.
Am posting to 1) to brag 2) to suggest anyone suffering from 100% CPU problems should look at working through the non-vital hardware list disabling one at a time - refer to these excellent articles/threads for further explanation .....
Svchost.exe Using 100% CPU Resources in Vista – Identify Causing Services My Digital Life
100% CPU Usage by NT Kernel & System - The Win Forums
Thanks to all for their help