AFAIK it's not possible to use Aero in VMWare, but they will try to implement it in future.
So what now? Install Vista for real? If I can help you in any way just tell me.
Ok, this doesent seem to be XnView specific problem because I found another program that has the same bug. Im not a programmer so I dont understand it, but I know it has something to do with Aero in Vista.