You're not alone. Two possibilities:
1. Disable the Office Plug-in in Norton Anti Virus.
2. Delete the Building Block.dotx file:
1. Close Word.
2. 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\Document Building Blocks).
3. Go into any subdirectories and delete your Building Blocks.dotx file.
4. Reboot Word will regenerate the file for you.