I needed to take a screen shot from a document and was having problems. This should be easy; Alt + PrtScn always did the trick in the past, giving a capture of everything on the screen. If necessary I could paste it into Paint and crop the chaff. Today, nothing.
I decided to try the Snipping Tool (Start > Accessories > Snipping Tool) which allows a much finer control in capturing images; you can draw a box around whatever you want and you're done. That's when I got this:
Windows has prevented this snip from being captured because it contains protected information. Close all protected documents and then try again.
Yikes, that sounds serious. Was this some new level of security inside Windows 7 that could sniff out personal or confidential information and keep bad guys from taking screen captures? Sounded iffy, but hey, who knows.
Short answer, "no". Long answer, "no, it's a bug".
It turns out that there is a known issue with Outlook(!) that causes this problem. Outlook makes use of something called Information Rights Management that allows a person to send a message that can only be read by people with a specific set of credentials. It's very useful for sending out confidential documents. It apparently can also flip a bit somewhere that causes tools like the Snipping Tool to think a confidential document is open, even when it isn't.
I closed and reopened Outlook and was able to perform the screen capture. Knowledge Base is your friend.
Disclosure: I am a design engineer with Microsoft.