Note: this entry has moved.I wish I new this before. 40 simultaneous connections!! Couple that with no HTTP Keep-Alives and it may actually work for you...
From that post (for my own reference, as it was down some days ago):
Alot of people were pinging me today about getting 403.9's when trying to access my website (Access Forbidden: Too many users are connected Internet Information Services).It turns out that the IIS on Windows XP comes configured out of the box for a maximum of 10 HTTP connections. What's worse, IE (actually URLMON) seems to use up 2 connections when hitting a website. I tried figuring out how to raise the connection limit, but there didn't seem to be a documented way of doing this on a non-server version of Windows. Fortunately, my good friend Geoff is the Architect of IIS and was able to help me out. Here's his tip:
This can be bumped up to 40 but not higher (40 is the hardcoded limit).
To do this, find the adsutil.vbs script (should be in c:\inetpub\AdminScripts or similar) and run the following command:
adsutil set w3svc/MaxConnections 40
You can also try to prevent individuals from hogging a bunch of connections and thereby blocking out other users. (IE will normally use 2 connections to a web site.) To do this, launch the IIS admin tool (under control panel / administrative tools), right click on your ‘default web site’ (under ‘web sites’ in the outline control), and mess with the settings in the “connections” part of the “web site” tab. Lowering the timeout (default is 5 min) will cause idle connections to get dropped quicker. Disabling keep alives will cause connections to disconnect immediately after the request is handled instead of waiting for new requests on the same connection. This is a little extreme but you probably don’t care that much about responsiveness, so you might want to just try that and see if it solves the problem.
Thanks Geoff! I hope I didn't cost the company too many W2K3 licenses by posting this :-)