Users of any a/v product, not just AVG, need to be aware that sometimes a virus definition update will contain an erroneous definition file that will target a Windows system file.
Just because an a/v says a Windows system file is detected as malicious does not mean it actually is, especially when it is a Windows system file. All a/v vendors have made this mistake at one time or another, but the real problem is that some users will not use a service such as Virus Total dot com to confirm the detection before deleting/quarantining the file; the fact that panic and haste and lack of awareness of a service such as VirusTotal sometimes gets an user into a bind that cannot be gotten out of.
So, do not panic if your a/v detects a file as malicious. Take your time to investigate first (use a second computer or Google the file in question from there if that would be safer).
Compounding the issue is the fact that deletion of any file will make it disappear forever. A case in point would be notepad.exe, a windows system file, you remove that by deleting, you will not be able to get notepad to run again no matter what you try to do.
If any question, always quarantine after, but use the link posted below. Do not reboot, (you may need that file to have Windows restart or operate properly) and upload and scan your file.
You can check with 40 + virus scanners here and make your best call here: https://www.virustotal.com/en/
If only your a/v alerts or even only one or two others, then likely to be a false positive and can be ignored. If, you did quarantine, you now have the option of restoring that file but you cannot restore if you choose to delete.
On Thursday morning, the protection programs of AVG incorrectly identified the Windows system file wintrust.dll as a trojan of type "Generic32.FJU". Under certain circumstances, the virus hunting software has also labelled programs as malware if they attempted to access the supposed trojan DLL. The solution is a virus signature update.
Only Windows XP systems were affected by the problem. Users who deleted the file from their system could not boot their computers any more. In this case, to help restore the system, boot it with the Rescue CD and take wintrust.dll from a still functioning system and copy that to C:\Windows\System32\. At least, according to AVG, the anti-virus software did not automatically delete or quarantine the wintrust.dll file, though other files will have to be moved back into place.
The company says it fixed the problem by 12:45 on the same day with updates to virus database number 567 for AVG 9 and 2012 editions and virus database number 6174 for the current 2013 edition.