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.