I've had this same irritating little problem for quite a while now. If you do a quick search on google you'll find many others have this problem too. I believe Microsoft are aware of this, as i remember reading a knowledge base article on the subject. However, the last time I checked Microsoft were advising users to ignore it as it won't have any adverse effects on Vista.
Frustrating answer I know, but at least you know it won't cause any harm!
Sorry I don't have any links for you to check as I haven't got them where I am now.