Hello!
For Elmah, we need to differentiate between two things:
First the http modules doing all the work of error logging, emailing...etc.
Second, the http handlers, displaying the error log page and other pages (rss...etc.)
I was having the same problem of 404 resource not found because I have a weird setup!
on my development machine, (windows 7, iis 7 ) elmah was working like a charm because the application pool was working in the integrated pipeline mode. In the production machine, however, the application was using the managed pipeline and I tried all my best to make elmah work but it was all useless...
I then got the idea of displaying the UI (error log page, rss, error detail,...) using regular aspx pages.
I downloaded the source code, made some changes (sorry Atif, I was forced to do this because I needed the quickest solution) and then in my application , I created a folder under which I created regular aspx pages which inherits from Elmah defined pages.
The page only contains one line (ex: for the detail page: <%@ Page Language="C#" Inherits ="Elmah.ErrorDetailPage"%>)
Now, I was able to run Elmah regardless of IIS 6/7 and it is working like a charm.. and It saved me from a big headache of correctly configuring http handlers and troubleshooting its work! additionally, configuring security is much simpler!
I don't know if the community is interested in this solution (If so, I am ready to post my full changes).
Hope that this gives you an idea on how to solve the problem in an alternative way (and if you need the modified dll with complete instructions on how to use it, just tell me!)