Microsoft plans to convert the 404 error pages on the microsoft.com domain, as well as other websites the company owns, to error pages with Live Search capabilities. This will allow Microsoft's customers to search for what they were looking for right away, instead of being stuck at a 404 error page that is equivalent to hitting a dead end.
Furthermore, Microsoft has also released a toolkit for webmasters who also want to transform their 404 pages to search-enabled pages. The 1.13MB Web Page Error Toolkit, a customizable ASP.net application that replaces the default error page on one's IIS server, is not limited to Live Search; web admins can choose whichever search engine they prefer.
Source:→ Ars Technica