Hi,
I've had this problem too, many times, since I got a 24" screen.
I didn't know the trick "Alt+Space>M[ove]>direction arrows", the only solution I've found then was to delete the Adobe PhotoShop config files. They are in C:/Documents and settings/your_user_name/Application Data/Adobe/Photoshop/your_ps_version/Adobe Photoshop Settings.
When you re-start PS, everything is re-initialized and every dialog box stands at the center of the screen (but you've lost your customization too 😞 )