Look into this KB nd if this doesn't fit the bill try the steps that follow :
http://support.microsoft.com/kb/925594
A simple Google gave me the following ...
Go to your user building blocks directory (\Documents and Settings\<user name>\Application Data\Microsoft\Document Building Blocks on Windows XP, \users\<user name>\app data\roaming\\Microsoft\Docume... Building Blocks).
Go into any subdirectories and delete your Building Blocks.dotx file.
Reboot Word will regenerate the file for you.
Note If you had saved any building blocks into your Building Blocks.dotx file you should be OK.