Antivir (Known today as
Avira) has a very loyal user base, good detection rates (at least reportedly so in the paid version, don't know about the free) and is definitely the one I would install if for any reason I couldn't use
Avast. Avast has the same protection/cleaning in the free version as it does in the Pro. The other free ones tend to have features, such as detection, somewhat crippled.
What Avast cripples is the ability to schedule scans.
Hardly matters. Nothing is ever found, anyway. The protection is good. The occasional manual scan is enough.