Do not open unexpected email attachments from known or unknown sources. A malicious e-mail attachment is one of the oldest, most effective, and easiest methods of infecting a computer. Always check the file extension of an e-mail attachment, because cyber-criminals can disguise a malicious attachment as a Microsoft Word document, PDF document, or other legitimate documents. Attachments ending with ".zip" or ".rar" are compressed files that should not be open if the sender is unknown to you, or you were not expecting the email message.