Sinan, Have a look @ this AVG article (http://avgclick.me/RepairAVG).
AVG Guru
I get an ''unknown error'' for two seconds when trying to update virus definitions. Then it continues to download update files but the error warning comes up again and it goes on in a loop.The update is never comleted.
Sinan,
I regret to hear about the inconvenience this may have caused. Please try to repair AVG Internet Security as per the instructions available in the link provided by Alan.
If you need anything else, do not hesitate to let us know.