Could you explain what happened? Could you not find such a program? If so, you need to uninstall it via Control Panel -> Programs.
I tried the programs own installer and also "Add/Remove Programs" and a third party uninstaller, in each case it came back saying uninstall could not proceed. I cannot repeat the process now as the Paint.Net is no longer listed in "Add/Remove Programs" and therefore presumably the registry and when I try to install Iget the above statement.