The anti-spam component of your AVG installation coughs loudly and marks the email as spam.
You say, "to heck with it, I WANT to see that photo of <insert name of latest celebrity> pushing her big toe into <insert name of favourite orifice>."
So, you open the email.
The AVG anti-phishing components coughs loudly and reminds you that this sure looks a lot like a phishing attempt.
There's a link in the email. "Ah ha!" you cry. "Here's the picture." You eagerly click the link.
Immediately AVG's linkscanner coughs loudly and says that the page you are attempting to visit appears to contain malware.
"Gosh," you think. "I've never seen what something like that could do to a PC. Forget the celebrity pic, this looks like much more fun."
So, in full and complete understanding of what is about to happen, you tell Linkscanner, "no, thanks anyway, but I will visit that site; thanks for your concern. After-all, I have AVG installed, what could possibly go wrong?"
Now the fizzer. Nothing happens. Not a sausage. Well, nothing you can see.
But congratulations, your PC is now the latest recruit in a very large botnet; expect it to be sending lots of spam (or other nefarious activities) in the very near future, using up all of your quota with your ISP, costing you lots in excessive download (or upload) fees.
Oh, and what went wrong with AVG? YOU did. In this scenario, you ignored three very clear warnings; each of which would saved you, had you paid attention.
Anti-malware software (whether from AVG or any other vendor) can only operate on the computer – it can't fix stupidity on the part of the user. Now, it's still possible that AVG (or the other vendors) could save you by detecting the infestation at some later point, but don't count on it.
Perhaps such people ought to be given a computer with no fixed disk at all; just a bootable optical disk with a bullet-proof OS on it.