Schvenn.net

Lately, this program seems to be the favourite of technicians to install as free virus protection. That may change in a year, because there are two or three programs that like to play a virtual hopscotch in that regard, but it's been more impressive recently because of it's effective boot-time scan and rootkit analysis.

AVG Free probably is the most used free anti-virus on the market & that's because it's very effective.

This anti-virus usually rates the highest in tests for effectiveness, but their nag screen is very annoying. There is a way to disable that though and it can be found: here.

This is one of the only anti-virus programs than lends itself to portability.

Here is another free anti-virus, but this one is great for portable drives, because it doesn't need to install. That makes if useful for infections that attempt to prevent themselves from being removed by causing anti-virus and anti-malware programs to fail when installing.