How to create your own custom 404 error page and handle redirect in SharePoint 2007 (MOSS)?

People alway ask how to use their own 404 file not found error page vs. the generic one from IE in MOSS environment. The following example catches the 404 error and sends users to a redirect page.

Here's the steps:

1. In your MOSS server, make a copy of
   %systemdrive%\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\1033\sps404.html
   and call it my404.html
2. Create a Virtual Directory in IIS under your MOSS root web application. For example /errors
3. Create your own redirect aspx page, for example /errors/my404redirect.aspx and code your redirect logic in there. This is a normal asp.net page.
4. In my404.html, make the following change:
   
   STSNavigate("/errors/my404redirect.aspx?oldUrl=" + requestedUrl);
   
5. Create a Console Application and insert the following code and run it in MOSS server

System.Uri webApplicationUri = new Uri(http://MyMOSSServer/);
SPWebApplication webApplication = SPWebApplication.Lookup(webApplicationUri);
webApplication.FileNotFoundPage = "my404.html"; //*note
webApplication.Update();

*Note: By default this is set to null. FileNotFoundPage needs to point to a html file that lives in %systemdrive%\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\1033. The file needs to be html only.

6. Now when you browse to a page that doesn't exist, you should expect to be brought to the redirected page.

*Another note:
In IE there's a "Show friendly HTTP error messages" setting which is ON by default in Internet Options->Advanced. With this setting on, sometimes your custom error page is not displayed. In order to override this setting, both my404.html and /errors/my404redirect.aspx from the above steps need to be larger than 512 bytes in size. Refer to the following KB about this setting: http://support.microsoft.com/kb/218155
* This seems to be working within a site collection context only, i.e. http://MyMOSSServer/sites/siteA if sites is a wildcard inclusion managed path and siteA doesn't exist in MOSS then this URL will NOT trigger the custom 404 error page set to SPWebApplication.FileNotFoundPage property.

Also look at SharePoint Smart 404 Feature in codeplex
http://www.codeplex.com/sharepointsmart404
It will give you advanced features