NOTE: Use 64bit editor ( Notepad2 x64 ) to change 64bit asp.net application's "applicationHost.config" !!
1. If current windows OS is 64bit ( win7 x64 or windows server 2008 ) , then 32bit editors ( notepad++ , visual studio ) can't actually change C:\Windows\System32\inetsrv\Config\applicationHost.config ; changing this file is actually changing C:\Windows\SystemWOW64\inetsrv\Config\applicationHost.config, which is configured for 32bit asp.net applications on 64bit OS platform !
2. So, to serve 64bit asp.net application we need access real "C:\Windows\System32\inetsrv\Config\applicationHost.config" , then we need a 64bit editor. "Notepad2 x64" is an ideal choice.