Because it can embed itself so deeply (in browsers, shortcuts, registry, startup folders, etc, etc), it is often not a simple matter to remove it. Often, multiple antimalware tools (such as AdwCleaner, Malwarebytes, HitmanPro, or even stronger) are needed to clean it out, and that implies a certain level of know-how for the user. The safest way for most users to clean it up is to get help from the expert volunteers at specialty websites that deal with this sort of thing routinely and can lead a user through the process, step by step. Reputable sites include DSLR Security Cleanup, Wilders, BleepingComputer, and others. The only thing to remember if you elect to get help at such sites is first, post with a clear title (eg: Only-search.com when I open Opera 25) and follow the directions they supply exactly as they request... and if you don't understand them, ask first and don't guess or assume.