Welcome to PPV

With the introduction of DirectX 10 and Windows Display Driver Model (WDDM) in Windows Vista, it is no longer necessary for an application to maintain a copy of its resources in system memory. Instead, the video memory manager makes sure that the content of every video memory allocation is maintained across display transitions. For compatibility reasons, Windows Vista emulates "device lost" for DirectX versions that are earlier than DirectX 10 to make sure that no application-visible API behavior changes.

More info: Nvidia, Microsoft

Download hotfix: 32bit, 64bit Vista