This issue is due to a bug in Windows XP.

Sometimes the Windows XP functionality used to access window images does not return an image of the windows contents. This can happen when the application that owns the window is busy doing something else, such as displaying a modal dialog or performing a lengthy operation. When this occurs, TopDesk will display the window contents as a black square.

This issue can be reduced by selecting "improved window image quality" in the Tiled Window Performance options. For more information refer to the TopDesk configuration documentation.