My experience with this problem:
Steps to Reproduce
I just experienced this using Windows Server 2008 R2 Standard and Visual Studio 2010 SP1.
I was accessing Visual Studio remotely through Citrix Receiver (from my computer, A, to B) and through Windows Remote Desktop (from B to C). In this set-up (chaining two remote sessions), I sometimes have stuck modifier keys.
In B, I had pressed Windows+Pause to access the System Properties window. (This may or may not be related; I suspect there could have been a stuck key press or something.)
Later on, when I opened Visual Studio on C, I had this problem. Note that I always use Ctrl+Shift+F to access the Find All window.
Solution
I solved it by pressing Ctrl+Scroll Lock as suggested in other answers.
Cause
As for why this happens, I did notice that if I press Ctrl+Break while the text cursor is in the Find Results window but before any results are found then the search stops with the same message. This indicates to me that it's related to a keyboard problem.
As reported by others, apparently it's a Windows bug. Here's a discussion about this.