I find it extremely annoying to see Amazon's "Amazon Assistant" banner
If you are sure that you haven't installed it yourself, then it looks like a "Bundled" or "Drive By" installation, attached to a 3rd party toolbar, browser or application that includes Amazon Assistant.
You should look at what other extensions/toolbars/applications you have recently installed.
In addition, check whether you have been infected by adware, spyware, etc.
About Amazon Assistant
The Amazon Assistant requires an Amazon account and an Internet connection, and is offered in a variety of forms on a variety of platforms, including as an extension for your web browser and a plug-in that may be included in or added to third party toolbars, browsers and other third party software applications.
Source About Amazon Assistant