AntiVirus System 2011 is a fake security software (rogue). It is from the same family as:
Security Inspector 2010,
AntiVirus Studio 2010,
Desktop Security 2010,
Total PC Defender 2010,
Desktop Defender 2010,
Contraviro,
UnVirex.
AntiVirus System 2011 displays a lot of disturbing warning messages pushing users to purchase a license.
If your PC is infected with
AntiVirus System 2011, use
MBAM to remove the infection.