I have seen multiple articles on redirecting Urls when the site has been redesigned or Url just changed to a standard format but I need to know how to manage when the Url has no correlation to the old one.

For instance, an old Url may have been `www.mysite.com/index.php?product=12` but there is no way to map that Url to the new site.

I don't want search engines to think that the page has broken so I assume the best thing to do is to 301 redirect to the home page but I am not sure how I would do that effectively. Would I just change the 404 error page to do a 301 to the home page?
Also, would that then cause issues with duplicate content via dofferent Urls?

Is it better to just not worry about these and let the search engines re-index the new Urls?

I am running IIS7 with Rewrite module and ASP.NET 2.

Thanks.