I happen to use AVG as well - and I believe this is a false positive. I have a few friends who use AVG as well and they all had the same message appear ..
so, wait for some official word from AVG
If you want to be safe verify you get this exact error:
AVG detection: Trojan horse Agent4.AQLZ C:\Windows\System32\shdocvw.dll - cannot be cleaned
If your virus detection is similar, then it's possible this is a false positive, else download Malwarebytes and scan again