Try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys or toolbars in your system.
AdwCleaner will mare sure to force the uninstall of the add-on. The error you encounter is related to the process that's running in the Task Manager. Just run AdwCleaner, press Scan, and then on Clean and it should be removed from your computer.