11
No, what Quest said is correct.
Go to System Admin>Blocks and add a new block (below your list of existing blocks). Make it visible in the Top Page only. Don't forget to click on the Submit button.
Once that's done, go to System Admin>Groups and click on the Modify link next to Anonymous. Then make sure the custom block is ticked, and all others you don't want shown are NOT ticked (but leave the Login block ticked, or you will not be able to log in.)
Then do the same thing to the Registered Users group, but DESELECT your custom block and make sure all the blocks you want visible are selected. You may have to select/deselect module access rights also (But DON'T give registered users module ADMIN rights, unless you REALLY need to )
That should do it.