The antivirus software AVG has caused a bit of a panic by warning its users that iTunes is a trojan horse that they are calling Small.BOG. This has led many iTunes users to uninstall the software. If one follows the AVG warning and quarantine the files iTunes.dll and iTunesRegistry.dll it will also make iTunes inoperable. Apple has said there is no problem with iTunes and there is no virus in their system.
It is totally possible that AVG has misdiagnosed the problem and there is a glitch in their own algorithm. This seems likely as other antivirus companies are not reporting the same problem. It is also possible that iTunes has changed something and although not a virus and not dangerous it has triggered AVG's Resident Shield.
If you are using AVG, as either a paid or free version, and use iTunes then to avoid the warnings popping up all the time just go to the Advanced Settings for the Resident Shield and click Exceptions. This is where you can specify files and folders that AVG can ignore. Add the path to your iTunes folder, usually C:\Program Files\iTunes\ and Save.
If you have accidentally quarantined the two dll files then just go to the Virus Vault section of AVG and you can recover them.
Do Not Panic!