This is for people who get the following error either immediately after you create a WSS site, or when you try to access a WSS site or SPS portal:

You are not authorized to view this page
You do not have permission to view this directory or page using the credentials that you supplied.
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Please try the following:

Contact the Web site administrator if you believe you should be able to view this directory or page.
Click the Refresh button to try again with different credentials.
HTTP Error 403 - Forbidden: Access is denied.
Internet Information Services (IIS)

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Technical Information (for support personnel)

Go to Microsoft Product Support Services and perform a title search for the words HTTP and 403.
Open IIS Help, which is accessible in IIS Manager (inetmgr), and search for topics titled About Security, Authentication, and About Custom Error Messages.

If you are using SPS, try to access http://<portalname>/default.aspx instead of http://<portalname>/.  If you can access the URL with the pagename, then the problem may be in your application pool settings.  Likewise, if you try to access a WSS site and get this error, see if you can add default.aspx to the URL.  If that works, same problem.  You might also get this error immediatelyt after you create a WSS site, at the point where you are supposed to pick the site template.

Fortunately, the fix is really easy.  You need to open up the IIS manager on the server, and the open the properties for your virtual server.  On the Home Directory tab, you probably need to change the Application Pool the site is running under.  There's probably one named SpsAppPool or something like that.  If there are several and you're unsure, try choosing them one by one, and trying to access the URL (the one without the default.aspx) after each change.

For SharePoint Technical Information, I recommend SharePoint Products & Technologies Resource Kit