Normal.mxt is the template where ArcMap stores all the file/menu/etc. customizations.
ArcMap reads this information on startup, but if it is missing (has been deleted), it will re-create it with default options, thus, will move all the panels and such back where they were initially.
Keep in mind, if you have installed third party tools and such, deleting the normal.mxt will likely break them, or at least un-do any customizations to the toolbar(s).
You should expect ArcMap to eventually corrupt it's normal.mxt (as it eventually does) and suddenly your toolbars/windows/menus/etc. either move or disappear.
This is such a common issue, I have a scheduled task that saves a copy of my normal.mxt every night. That way, when I open a map document and see that the template is corrupted yet again, I just close ArcMap, and replace the normal.mxt with the last 'good' copy, then restart ArcMap and you are back where you were before it decided to corrupt.
R_
