Copy from http://our.umbraco.org/forum/using/ui-questions/8244-IIS7--404:
Basically, you need to add
<location path="Site Description">
<system.webServer>
<httpErrors existingResponse="PassThrough" />
</system.webServer>
</location>
to your applicationHost.config file where "Site Description" is the name of your site in IIS7.
The applicationHost.config file is located in: system32\inetsrv\config
Edit:
As stated in the comments if this answer, you should add this section in your web.config instead which is way better, you should always avoid altering config files outside your own application that may affect other applications.