and the solution...
go to http://www.dougknox.com/xp/file_assoc.htm and download the EXE File Association Fix.
Oh so easy in the end. Just took me four hours to get there.
I spent a couple of hours this morning fighting off a virus. With success, but the fix has corrupted a key file, rundll32.
I have had a search on the net and have tried copying the file into place in the appropriate folder. I have also copied it there from my windows startup disk. No good.
I can see the file in the folder, but it is not working.
It is a big issue as I am not able to run any programmes now, including instructions from the start-run command line. I can get on the net, that is about all.
Is there anyting obvious I am doing wrong?