Hi Ego,
I have finally had some success. I tried uninstalling one at a time but everytime I re - installed the framework version I had uninstalled I got the same error message.
I decided to go the whole hog and used DotNetFx and completely uninstalled all versions which were installed. I then ran CCleaner Registry cleaner.
Went to Microsoft Web Site - Custom Installs and after it did scan stated I needed to install .Net Framework 4 Client Profile first before installing any other Frameworks. I then downloaded eleven (11) .Net Framework 4 updates. Then Installed .Net Framework 3.5 SP1 ( which included v 2.0 & 3.0 ) after installing these some more automatic updates were downloaded.
I then ran Paint.net installer and behold it worked and installed.
I have not installed v 1.1 it shows as being available at the Microsoft site if I want to download it. I do not know what this is for, so will not be downloading this at present.
It has took a while but at least I am up and running again.
I can now get on with doing something else like enjoying the British Summertime!
Best Regards.