Fake tech support scams are more and more popular these days. These ploys typically happen when browsing and have a very convincing look, along with a phone number to reach “helpful” staff. In some cases you may also receive an unsolicited call from “Technical Support”, offering to fix your computer.
DO NOT give these people access to your computer.
DO NOT provide usernames, passwords or any other personal information.
DO NOT give these people your credit card number.
More information here:
http://www.thesafemac.com/tech-support-scam-pop-ups/