Make sure your IIS authentication is set for anonymous. Make sure the user that is the identity on the app pool has read permission on all files and folders in the web folder and full control on /Data and /App_Data and their subfolders. It sounds like some kind of file system permission problem. Check also the mojoPortal log for clues under Administration > System Log
View the source of the page and follow the link for the css handler to find if it is returning an error or returning css.