Nagware window preventing my setup

I'm trying to set up an old Mac Mini with a new keyboard and mouse. I have logged in ok with the wired keyboard, but a window has popped up from an app that was on trial, wanting me to buy a license or quit. I can't do either as the wireless mouse isn't paired, but the window is obscuring the system preferences window so I can't pair the mouse using the keyboard. Is there any way to close this window without buying a wired mouse just for this one little click?

I can't even be sure it's Mavericks this thing is running because this nagware windowis over the About This Mac window as well.