Have you had a 2-screen setup at any point?
The few times I have heard this happening is when the window is actually appearing off-screen. The program stores its last position and uses that as the start position next time you open it. These are stored in the registry and if they have got messed up (or you previously opened it on a second screen)it will be positioned according to these

Best bet is to download the "registry cleaner/editor..." listed under the Specials on my front page. Find DXTBmp in the list and check the entries for "Top" and "Left" as these control the position of the top-left corner of the window. Change them to values well within your screen area (0 and 0 will put it at the top-left of the screen) and then run DXTBmp