No you don't have a virus, WebAdvisor was merely asking, as it will every time a new version installs, or if your browser undergoes a major update, if you would like to restrict your searches using Secure Search. Doing so makes Yahoo Search the default search engine in all your browsers as Yahoo is the only one to sign on to that feature, thus far.
You can always undo it in WebAdvisor's settings. Just click to the right of the cogwheel icon near the top right of any page in Internet Explorer and select WebAdvisor - settings and undo it there for all browsers. (The setting in IE controls the setting in all browsers and while there you can alter the other settings).
As you've said, WebAdvisor is already installed and enabled - this pop-up (sometimes called a "toast") is asking how you want WebAdvisor configured. Specifically, it's asking you if you want to enable SecureSearch (which it will do every few months).
A couple of minor corrections on what Peter has said;
Configuring SecureSearch as the default search engine: Several months ago, we changed WebAdvisor so you could configure a different search engine in each browser - Internet Explorer, Firefox, and Chrome - because many people wanted that flexibility. Consequently, the pop-up setting is also configurable per browser.
Configuring another Search Engine, other than SecureSearch: We do not track what you had previously set as your default Search Engine, so you will need to select another Search Engine inside the settings for each browser: These are where I can change the settings on my system which runs the latest browser versions (as of today, these are IE 11.0.9600.18125, Firefox 43.0.2, Chrome 47.0.2526.106 m)
In Chrome, go to Settings > Search > Manage search engines....
In Firefox, go to Options > Search > Default Search Engine
In Internet Explorer, go to Tools > Manage Add-ons > Search Providers