Try closing Word, then navigating to:
Code:
C:\Users\%Username%\AppData\Roaming\Microsoft\Templates\
and renaming the file named Normal.dotm. When Word restarts it will create a new one. Any new document created should now have Word's default set-up (which is not Helvetica triple spaced). If your other documents now behave normally, that's because they have the 'automatically update document Styles' attribute set (see under Developer|Document Templates|Template). You might want to uncheck that option so the documents can retain their own Style definitions.