If you all of a sudden notice that you cannot drag&drop files inside your Visual Studio from outside on Windows 7, for example, from Windows Explorer, it might be the case that you are running VS under Admin user rights. In this case it won't allow you drag&drop-ing files inside it.
The solution is simple.
Turn off User Account Control. Go to Control Panel -> User Accounts -> Change User account Control settings. Drag the slider all the way down and reboot.
The thing is that by design when you run VS under admin privileges it starts in different security context and thus disallows dragging files from Windows Explorer. Disabling UAC fixes that. Unfortunately doing that you will also no longer see warnings when you try to run unknown (possibly malicious) software. But that's the price to pay when you have to run VS as admin.