Strange experience of permission changed

Under WinXP, I made a copy of some of web application files to another folder, and copied them back to the folder where the web application is in.

Supprisely, the permission for the webapplicaiton folder was changed!! then you got a access denied error:


Access is denied.
Description: An error occurred while accessing the resources required to serve this request. You might not have permission to view the requested resources.

Error message 401.3: You do not have permission to view this directory or page using the credentials you supplied (access denied due to ACLs). Ask the Web server's administrator to give you access to 'C:\Websites\globalGateway2\bespoke\ISA\Forms\frmLogin.aspx'.