Many thanks for your recommendation. This anti adware program appears to have done the trick. 120 items were Quarantined in 2 batches. They are listed under the name "Alexa". The home page is now changeable again.

I didn't think a free edition would allow you to quarantine/remove infections. Apparantly McAfee does list this threat but didn't do me much good. It is interesting to see how different anti Spyware/Adware programs catch the different problems. I think most of the rubbish ends up in the Tempory Internet Files but the most serious things get elsewhere.