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Welcome to WebmasterWorld.
Are you hiding the panel by setting the Panel.Visible property server side, or are you presumably doing something like display:none in your CSS?
In either case you could wrap the area in question in a container of some sort.. table.. whatever.. that has a static height and width defined. Then, whether you hide or show the content, the space occupied on the page will be the same regardless. If that's still a bit fuzzy, post some code and I'm sure we'll be able to get you where you want to be.
:)
-Mark