Trojan that notifies users of Windows Security update is a rogue advertising adware. If you manage to remove Antimalware Defender (AntimalwareDefender) related trojan you will not need to get rid of Antimalware Defender adware. The   alert produced by Windows Security trojan reads as follows: “Antimalware security update for Windows XP (KB961118) Size: 433KB This critical update will install [...]

Source: http://hands-oncorp.com/2010/02/01/antimalware-defender-removal-instructions/