Thanks for your answer richc46.
I went through the clean boot procedure, which made no difference. I always keep a close eye on msconfig and the only startup items were ATI drivers
anyway. Safe mode : yes. Started..
I began a full scan with MS Security Essentials but after 45 minutes it seems a full scan might take 6-8 hours. Malwarebytes will not install. I get the same message:
"The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log or use the command line sxtrace tool for more detail."
TBH I think a virus is a very low possibility. I keep MSSE up to date and had Malwarebytes installed until recently. I indulge in very low risk behaviour. Even my web browsing is with Firefox with adblock plus, no-script etc
UPDATE : I am commencing to format the Windows partition and reinstall 7. It should only take six hours or so to reinstalll everything.