Create a duplicate of your current site definition in Dreamweaver:
Site > Manage Sites > (select your site) > Click 'Duplicate' > Click 'Edit'
Change the local and testing site root directories to your PowerCMS directory rather than your site root directory, and save it as YourSite (CMS). Ignore DW warning messages if there are any.
Use this site definition to make Power CMS template changes, CMS HTMLEditor changes etc., and use your normal site definition 'YourSite' for any changes to the rest of your site.