reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
Then close Dreamweaver and Delete the Dreamweaver Cache file from the users configuration folder:
http://www.webassist.com/tutorials/Common-installation-issues
you may also want to try disabling the preview, go to WebAssist -> Preferences and in the preview section, elect the Disable preview option. you will still be able to see the preview if you click the preview place holder.