After downloading a few free programs I discovered a few unwelcome additions to my favourites. They reinstall themselves after deletion. Spybot and Panda antivirus can't spot them. My Task Manager was also disabled, I suppose a the same time. Any good advice, apart ffrom "you didn't want to do that, did you?"?
safe mode + system restore seems to have done the trick. Previous attempts with system restore, not in safe mode, told me there were no changes to effect. Many thanks for your help. If you ever need viagra etc I might be able to point you towards a site...perhaps not!
vog led me to malwareremoval.com, where I downloaded trojanhunter. This found 2 trojans lurking, and eliminated them. So far so good. Still no real joy with task manager, which just comes up as a single page.