How do I unlock a file that was locked by AVG Antivirus?

A file on my computer was labeled as a threat by AVG Antivirus. I chose "ignore" but the file remains inaccessible. I cannot copy, move, delete, or execute the file. I have shut down AVG and even uninstalled it. I have removed the drive and connected it to a computer without antivirus software and the file remains locked. It must be something in the file system.

Are there any ways to unlock this file? Don't ask me what the file is, That's irrelevant. And, yes, I am certain that the file is not a threat.

Thanks for any help you can provide!