Is there a way I can modify the 404 Error page.

We are cutting over from our old HelpDesk system to yours.  I plan on pointing the IP address in DNS of our old sever to the new server so the old links will route to the new server.  Problem is when doing so, the old links will issue a 404 error on the new server.  I would like to modify the 404 page or have it somehow automatically redirect to the default page for the user to sign-in or better yet for single sign-on to auto authenticate the user.  I have everything working the way I want it, with single sign-on if you go to the server directly, I just want to give the users a method of signing into the new system if they happen to use old links.

Thanks.  Arnie