It´s a bug in the signature.
AVG is still working on a update.
Known problems only at XP and AVG 9.
I want to add this:
few AVG 2012 BE on some of our win xp machines notified the same alert after a post-update scan that ran this morning.
In those cases there were about 20-30 threat notifications (some were repetitions), all of them referencing different processes handling wintrust.dll and automatically putting the library into the whitelist.
The signatures are the same from 9 to 2013.
So there are the same errors on different versions.
They told me that they know only problems from XP and AVG 9.
But we had a customer who has the problem on at W2K8 R2 TS-Server with 2012.
So every actual AVG version has the same problem.