Using VSTO 2005 and Office 2003 you can create an Action Pane (see video). This is a document level object, which mens that it is attached and visible only when the document is open. Now in using VSTO 2005 SE and Office 2007 you can create CustomTaskPanes. These are application level in scope so it is visible as long as the applicatin is open. On kathleen's blog she has created a video which walks you through the steps needed to convert your ActionPane to a CustomTaskPane.
Converting an Actions Pane Solution to a Custom Task Pane Solution
Look for more VSTO for Mere Mortal videos soon and be sure to grab a copy of our book.