All "scareware" (software that tells you stuff like "WARNING! CLICK THIS LINK OR YOUR COMPUTER WILL DIE") are to be avoided. If you get that, close the site you're going on, that means they're scammers. On that note, avoid ALL applications that are related to "cleaning your computer" or "fixing problems", especially if you're on Mac. I think they're not *that* bad on Windows, but on Mac it's very, very harmful. Avoid and immediately uninstall Mackeeper.