Also, download their rescue CD, burn it to a blank CD, then boot with it, letting it scan your Windows drive for viruses without having booted from Windows.
“Avira antivirus”
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Most AVs are good enough, viruses aren’t the big threat they used to be - rootkits, botware and ransomware are bigger concerns these days.
Bitdefender also provides a free scanner that you can burn on CD or USB stick:
http://www.bitdefender.com/support/how-to-create-a-bitdefender-rescue-cd-627.html
AV Comparatives is a good reference site for those considering the effectiveness of misc AVs.
http://chart.av-comparatives.org/chart1.php