Being able to open the Building Blocks.dotx file is not the issue. Repeatedly telling us you can do so contributes nothing to the solution, which is likely to require you to:
• move, delete or rename the Building Blocks.dotx file; and/or
• repair your Office installation.
Both approaches are mentioned in the link I provided. Granted, the FixIt is for Office 2007, but the other approaches apply equally to Office 2010.

Other possible location for the Building Blocks.dotx file are:
C:\Documents and Settings\%Username%\Application Data\Microsoft\Document Building Blocks\1033\Building Blocks.dotx
C:\Documents and Settings\%Username%\Application Data\Microsoft\Templates\LiveContent\User\Word Document Building Blocks\1033\Building Blocks.dotx

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]