Changing SharePoint 2010 Access Denied Page

I'm looking at the best method for changing the Access Denied (and possibly other error message screens)  for SharePoint 2010.
I've done some reading up and many have talked about changing the _layout/AccessDenied.aspx however being a novice at SharePoint I am unable to locate such file, furthermore many have mentioned its not recommended to change this file. I am wondering then whether its possible to insert maybe JavaScript to catch the error and redirect it to my custom page.

Here is the issue I am having it shows a iframe that is on an open page however the sharepoint calendar is locked down, but I dont want the access denied page to be shown to unauthorised users.


Any help will be great :)