AVG maker Grisoft has long had a popular free (non-commercial) anti-virus program for Windows. Up until now however it was based on old code, and was relatively slow to receive pattern file updates.
That has just changed with the release of the new version 7 release of AVG free anti-Virus. If you are using the old version, you will need to update before the end of the year as updates will stop for previous versions at that time.
I run two anti-Virus apps on my Windows machines, because one often picks up things that the other would miss, and my choice of an alternate program is the Open Source ClamWin anti-Virus. ClamWin is small and fast so there is no noticeable slowdown when running both programs in the background.
Franki