Error: HTTP 404: File Not Found in MOSS 2007 (SharePoint)

Error:  

HTTP 404: File Not Found. 

(Or)

SharePoint site can not be opened in SharePoint Designer.

Reason:

Usually this error will occur due to the below reasons:

1. If the portal is not attached with "Central Administration" portal.  i.e. The portal will be running        in IIS. But the Portal site may not be created in "Central Admin". 

2. The Port may not be associated with SSP.

3. Identity of the IIS application pool may be different. or the Application Pool Identity password may be changed. 

Solution:

1. Create a new "Web application" in the same port with the previous credentials & with the same database name through the "Application Management".  And don't create "Site Collection". (This is applicable, if the problem is due to Reason 1")

2. Associate the Port (Web application) with SSP, if the reason is 2.

3. Go to IIS and open the properties of the Application pool for this specific port and update the username,password in the "Identity" tab.

Important: 

Don't forget to restart the IIS. Else, your master pages will not take effect in the "Application pages".