Custom 404 works but throws up a pop up window

[misterm2008]

Hi

I have added the following line to my .htaccess

ErrorDocument 404 /404.php

When I type a url for my website that doesn't exist, it does the job and send the user to 404.php.

The problem is that it also throws up 2 windows, each with the header:

Error: the server returned the following HTTP status: 404
Received:

followed by the source code of the 404.php file.

Why does the pop up window appear and is there a way to turn off the the pop up window?

Many thanks

Mr M

[bill]

What's in 404.php?

What type of server?