Cybercriminals often use publicly available phone directories, so they might know your name and other personal information when they call you. They might even guess what operating system you're using. Once they've gained your trust, they might ask for your user name and password or ask you to go to a legitimate website (such as www.ammyy.com) to install software that will let them access your computer to fix it. Once you do this, your computer and your personal information are vulnerable.