All that's happening is Windows is trying to load this file on startup, but it has been removed, either by you or an anti-malware.
That said, it looks like you'll need to manually remove the registry entry that refers to this file. You can try ccleaner (free) or do it yourself.
To do it yourself, open regedit (Start > Run > type 'regedit' (no quotes) and hit enter) and go to Edit > Find, type M3PLUGIN.DLL into the search box and start searching for it, when you find it, right-click the entry and select delete. Keep searching until regedit says it can't find it anymore and reboot.
In the future, click only the X on the top right corner of pop-ups you receive while browsing the web, and for the love of Pete, do not install anything that offers itself via a pop-up!