Success at last! I got a little worried when I discovered that even regedit was unable to alter the contents of the registry so realised that something must be blocking it. The reason for the problem, I eventually found, was Adwatch. When this active spyware blocker which is supplied with Ad-Aware SE+ was disabled, all problems disappeared!
The annoying thing was that I have used this facility for ages and it seems to do a good job in stopping the various bits of malware which otherwise get into my machine. Furthermore, my daughter purchased an identical Acer a day or so after I bought Mrs. P's. I installed Ad-Aware SE+ with Adwatch on this for her as well and she has no problems at all changing her Internet Explorer home page!
Many thanks again to everybody who kindly made suggestions to sort out my problem.
P.P.