IIS Redirect from Bigboot Software records the failing pages and initially sends them to your home page. You can then using a simple application to painlessly point them to the correct page. You don't have to remember all the links you used to have, as IIS Redirect will capture them. Google does not drop the old links as it thinks the pages are still valid. When the site is re-indexed by the search engines the old links remain. You end up with more search engine links which means more hits to your site.
||Free to try
Windows Server 2008