You should not install it again..
Boot the computer into safe mode and try the uninstall again.
No luck you will have to manually uninstall which is a pain. Delete the files and open regedit and delete the registry entries.. better still use the system restore to go back to a previous system where you had no 4.2 installed..
Or just ignore it and install the latest 5.6.1 and if it gives trouble well do a clean install of windows.