Your best bet in IIS6 is ISAPI Rewrite. It's meant for this exact situation. When you move to IIS7, there's built in support with URL Rewrite.
Since this is IIS6, web.config won't be able to redirect a .asp page (non-managed code) unless you map .asp to aspnet_isapi.dll instead of asp.dll (but then it won't process the pages the same). So IIS6 can't cleanly natively do this by itself.
What you can do is in your details.asp page watch for item_id=something and do a Response.redirect.
Or you can do this in global.asa Application_BeginRequest if you want it more generic and for multiple pages.