A customer of mine experienced a weird issue where they were unable to insert a page number in Microsoft Word 2007. When they went to insert page number in Word they received "Save Selection to Page Number Gallery".
After using system internals process monitor I discovered that Word references a file called "Building Blocks.dotx" to create the page number.
On Windows XP/Server 2003 this file is located under:
C:\Documents and Settings\username\Application Data\Microsoft\Document Building Blocks\1033
In Vista/7/2008 this file is located under:
C:\Users\username\AppData\Roaming\Microsoft\Document Building Blocks\1033
I copied Building Blocks.dotx from another computer with Word 2007 installed replacing the file in my user profile. This resolved my issue.
Note: I believe that this resolution will also work for Word 2010.