Yahoo Is Ignoring Stardard 404 for page not found... Why?

Yahoo keeps accumulating old bogus pages even though I told IIS to use the standard 404 page for page not found.

C:\WINNT\help\iisHelp\common\404b.htm

I've used the page tester tool "WebBug" and it returns the following "HEAD" when sent to one of our bad pages...

HTTP/1.1 404 Not Found
Content-Length: 1635
Content-Type: text/html
Server: Microsoft-IIS/6.0
Date: Mon, 18 Dec 2006 17:49:50 GMT
Connection: close

Has anyone else had this problem and know of a resolution. Yahoo seems to respond well to 301's and get rid of the old page, but it never seems to work well with 404's.

Thanks.
Chris.

needs to be set to:

Response.AddHeader "404","File Not Found"
Response.Status = "404 File Not Found"

and that should do it in Classic ASP.