Does it open even when you create a new project? How about finding a file and renaming it does this problem help? Also, did you try to backup and delete the contents of C:\Users\<your_username>\AppData\Local\Microsoft\VisualStudio\ ?
Update 1 based on comments:
So, when you renamed all your A.XAMLs to B.XAMLs, do you get an A.XAML message or get a B.XAML file? Do you have many instances of the A.XAML file on your computer? If there are a lot of them, rename them differently so that you can subsequently identify the perpetrators. Have you tried to "repair" VisualStudio using the "Add Remove Program" in the control panel? Can you find the A.XAML line in Regedit.exe? I think it would be useful if you created a fictitious project and asked a colleague or friend to open it on your computer? (It would be helpful to get a sample of this dummy project for troubleshooting, whether it is a project, or an IDE or OS that is the culprit).
Update 2 based on additional comments:
Recreating the project .SUO file fixed the problem. For example, renaming the .SUO file to .BAK and reopening the project in Visual Studio.
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