Free Real-Time Antivirus Software

Avira Free
https://www.avira.com/en/free-antivirus-windows

AVG Free
http://free.avg.com/us-en/free-antivirus-download

Avast
https://www.avast.com/free-antivirus-download

Bitdefender Free
https://www.bitdefender.com/solutions/free.html

Fortinet Client
http://forticlient.com/

Comodo Antivirus
https://antivirus.comodo.com

Freeware On-Demand Virus Scanner

Avira Virus Cleaner
http://install.avira-update.com

Malwarebytes
http://www.malwarebytes.org/free/

Avira Rescue CD
http://www.avira.com/en/download/product/avira-rescue-system

TrendMicro Online Scanner
http://housecall.trendmicro.com/

Kaspersky
https://usa.kaspersky.com/free-virus-scan

ESET Online Scanner
https://www.eset.com/us/home/online-scanner/