If you see the pop-up below while on the internet on your Mac, DO NOT call the number. If you call the number, DO NOT give the “technician” remote access to your Mac. This is a scam to get control of your computer and have you sign up for technical support.
It is important to understand that no website can scan your computer for malware or suspicious activity. Mac OS X will never display such a message within your web browser.
Press Command-Option-Esc to display the Force Quit Applications window. Force quit your browser to make the message go away. Restart your computer.
Read a great explanation of this scam here.