Thank you for posting your experience, but the question is how it was installed on the computer. It could have been an ad, could have come with a program you downloaded. See this article which explains how it could have been installed: Watch out for fake virus alerts.
Also, have you checked with the Windows Update website to check if the computer has the latest (security) updtaes installed? What is the version # of the McAfee DAT, antivirus signature, file?