On top of the dropdown box containg the error message the heading says "Owl Orphanage". Googling this seems to indicate that it is a problem with Photoshop and not Windows. Once I've repeatedly hit the delete key the message disappears and everything works normally. This seems to tell me that CS5 and Windows are not properly talking to each other, rather than the file is missing. What I don't know is which end of the chain the problem lies.