My Font book opens every time I log in. Is it a virus?
It's very unlikely to be a virus, more likely a corrupt configuration file.

If you mean the Font Book app is actually opening, then the first thing to check would be System Preferences > Users &amp; Groups. Select your user account and click the Login Items tab. If Font Book appears in the list, select it and click the "-" button to remove it.

If you mean it's just appearing in your dock, but the app isn't actually opening, you can simply drag it out of the dock and wait until the "Remove" tooltip appears.