Assuming you download the iTunes installer from Apple directly, not some third-party web site, almost certainly AVG is misidentifying some iTunes component as being a virus. This misattribution is not, from what I've read, uncommon with AVG; I've seen reports of this mistaken trojan definition going back to 2009. AVG probably needs to issue a virus update to correct this.
For now, try this:
Open AVG and go to Tools > Advanced settings. Expand Resident Shield > Exceptions > Add Path > navigate to your iTunes program folder and add it as an exception. That should stop the constant erroneous error messages.
A temporary solution is to put the C:\Program Files\iTunes path in the exception list of AVG's resident shield (better than disabling the shield alltogether). That is, if you trust this to be a false postive. AVG main window, Tools/Advanced settings, Resident Shield, Exceptions, Add Path. Once you click Apply, iTunes starts normally (at least on my PC, Windows Vista Ultimate). Note: it's your responsibily to bypass this virus alert. Good luck!
I am having the same exact problem. I used it fine right after the update a couple of days ago, but now it is having this problem. AVG let me move the threat to the Virus Vault, but it would still detect the Torjan Horse everytime I tried to open it and even then it still wouldn't let me open iTunes. It gives me a message saying that something it wrong with it and suggests I reinstall the program.I uninstalled and reinstalled it and it still had the same two problems.
I tried Dave Sawyer's suggestion and AVG stopped giving me threat detection messages, but iTunes still won't won't. I am trying to reinstall it again right now, but I don't know what I am going to do if it still won't let me open the program. Any advice?
As of NOW AVG have fixed this "error" with an update.
So the procedure is
1. Update AVG to the latest definitions
2. REMOVE the iTunes from the exceptions list as posted by Dave Sawyer
3. Repair iTunes by running the installer again, selecting "Repair" instead of remove.
It is always better to let AVG manage the exception as this protects your iTunes folder from any future virus.