Does the wa_preview.js file exist if you browse to:
C:\Users\<your User Name>\AppData\Roaming\Adobe\Dreamweaver 9\Configuration\Shared\WebAssist\Library\
Where <your User Name> is the name you use to log into windows.
If the file exists at that location, in Dreamweaver, go to window -> Workspace layouts -> Reset '<Layout Name>'. where <Layout Name> is name of your workspace layout. Then restart Dreamweaver.