When I run Sharepoint Designer it automatically connects to MOSS using my windows credentials which are configured as a normal user . I then got a ‘you do not have permission’ error and it pops up a login dialog for me to provide the correct credentials.
here is a simple solution:
right click on the SharePoint Designer .exe (C:\Program Files\Microsoft Office\Office12\SPDESIGN.EXE) and select “Run As…” and use the credentials you want..