My following answer already prepared in case you have already uninstalled UltraCompare was:
If you have already uninstalled UltraCompare Professional completely and UltraEdit still thinks, the professional version is installed, you can copy following lines to a text file, save it with a name like "uc_prof_remove.reg" (extension reg is important) and double click on it to import it to your registry.
[-HKEY_CURRENT_USER\Software\IDM Computer Solutions\UltraCompare Pro]
[-HKEY_CURRENT_USER\Software\IDM Computer Solutions\UltraCompare Pro 3]
This will delete the specified keys and UltraEdit will use again UltraCompare Lite. The bold highlighted key is the key used by UltraEdit and UEStudio to determine if UltraCompare Professional is installed or not.
Best regards from Austria