I have a website which built with Drupal and the page not found redirects the users to:
Page not found http://www.sitename.com/node/126
When I check the error code for this page, it has a "success 200 OK".
Now what changes should I do to yield a 404 error so the users who clicked on this broken link and landed on this page will get a 404 error, and at same time Google will know it's a 404 error and prevent the link from being indexed?
Looking at the HTTP header for the URL you provided (http://www.sitename.com/node/126) does provide an HTTP 404 error response from the server, and not a "200 OK status", which results in 404 error pages in Chrome, Firefox, IE, and Opera browsers.
You can test this by pasting the URL in here: View HTTP Request and Response Header.
I'm not sure if you're asking how to configure your web server (IIS 6.0) to display 404 errors for pages not found, but it appears to be doing this already as is evident by requesting different non-existent URL's (e.g., http://www.sitename.com/node/127).
So everything looks fine, and as Google crawls your site, it will receive the 404 error response from your server for any links not found.
