Totally unsecure
Some good points made here:
http://www.thedvdforums.com/forums/showthread.php?t=512083
"It's easy to detect if AVG is prefetching a page, this means that malicious sites can actually redirect the AVG requests to a clean page that contains no malware. As a result AVG will report that the page is safe and allow you to click through to it - when in fact all it saw was a dummy page and the real page could be infected with many many exploits.
It wastes bandwidth and creates fake DoS attacks on websites. I spent a number of days trying to track down the cause of server issues on CD Times - finally with help we managed to determine that it was AVG creating what amounts to a DoS attack from multiple zombie machines. The software had hit upon a custom 404 page and in the period of 1 month I have had millions of requests - hundreds of thousands per day - for a page that doesn't even exist. The frequency of these requests resulted in the server becoming unstable and caused the site's monthly bandwidth use to increase by more than 20 times."
Biting the hand that feeds IT
