I don't have Spy++ but I did find Winspector Spy: http://www.windows-spy.com/ManipUni said:Lauch Microsoft Spy++ and use the "Find Window" tool. Drop it over the window and discover who is putting transparent windows in the middle of the screen.
It turns out that it's coming from Foobar2000, possibly foo_title. I just shut down Foobar2000, and the symptoms have disappeared. I removed dotnet_title.dll from my components folder, and it no longer appears when I run Foobar2000.