AVG Antivirus Software from Grisoft

AVG is probably the most popular free download on the Internet.  It is a full scale antivirus software.  I have used it for years, the free version.  It downloads, installs, updates, and regularly scans your PC.  The scanning will slow down your system.  So choose a time when the PC is not in use.
http://free.avg.com/us-en/download-avg-anti-virus-free