Strange Sub-window In WinMail

Contrary to others I love Vista!Meanwhile I have 3 machines running under Vista (my desktop, my TabletPC and my wife's desktop). What bothers me most at present:
For app. 2 weeks - out if a sudden - on my desktop (and only there) I'm having an unseen phenomenon with the screen layout:

Below the normal parts of the WinMail-window (columns for the folders[left side] and the headers preview and the messages preview [right,upper and lower half]) appears a sub-window with a scrollbar which stretches across the whole WinMail window. This sub-windows always is totally empty (no contents at all).

As this description might be unclear, pls see:
http://www.hera.hs-heilbronn.de/Rt/WinMailProbs.jpg

I cab drag this sub-window bigger and smaller. But there always remain a minimal height determined by the "Up" and "Down" scroll-button (arrows).

I checked literally *everything* to get rid of this totally useless(empty) sub-window. To no avail An I already asked in the German Vista NGs. To no avail either {siiiigh}