av.exe is a malicious process, which belongs to a big group of viruses that are all clones of each other. These programs are XP Antivirus Pro 2012, Vista Internet Security 2010, Win 7 Guardian, XP Defender 2013 and many other ones that all share identical GUIs and seek to steal the money from their victims. Av.exe is a critical process for these threats because it helps to launch certain parasite components when needed. Besides, it has also been reported to change user's start-page and disable legitimate security software. Be aware that its presence indicates that your computer is infected with a particular threat and you must terminate av.exe as soon as possible.
Mostly, Av.exe is dropped in the %AppData% folder (typical location C:\Documents and Settings\[username]\Local Settings\Application Data). However, you shouuld also note that this actually may be a fully legitimate part of the operating system or legitimate software. We recommend carefully checking the file before deleting it from your computer.