In Chrome, check that you don't have any extensions you don't expect (menu -> Tools -> Extensions).

In IE, check that you don't have any plugins that you don't expect (Cog -> Manage Add-ons).

I don't have Firefox installed on this PC.

Also, use SysInternals Autoruns, which will display a list of stuff loaded at startup. In particular, since this is affecting your browser (if it's IE), look in the Internet Explorer tab.

Note that most of this stuff is benign, so don't go disabling or uninstalling it without checking first.