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Error 0x2733 when making a request to site

We are hosting an ASP-based intranet site on IIS 5 & Win2k server. The server also hosts other sites on different port numbers - this one being port 1000. The server is based in the UK and some of our users in the US (but by no means all) are unable to access the site (incidentally they can access some other sites on the same server and can ping the server no problem). Upon further investigation I ran Microsoft's WFETCH utility to see what was being returned. I get the following error when making a request to the site: Error 0x2733 (A Non-Blocking Socket operation could not be completed immediately.): Connect().

Now the problem is not specific to this port number - I get the same problem accessing other ports (with different applications hosted). Some ports are fine and they can access the application no problems on a development server. It is worth noting that majority of users have no problems what so ever accessing the site.

E.g. ports that are fine: 6075,6051,5702,5701,6005
e.g. ports that return error: 801,105,1000,1001

I have been able to discover that the error is a Winsock error 10035 (WSAEWOULDBLOCK) and occurs frequently as a 'try again later' status code when using Winsock API calls directly. However I have not been able to figure out why this should happen here and the problem occurs all the time via the browser for this particular group of users.

As a workaround we have copied the site to a port that works (10000) but obviously we're nervous that that this may work for one lot of users but not all. Any ideas what's causing this?
Since you're having problems with only a fraction of your user base, it's probably not your fault, but you are stuck with fixing it. Someone between your users and your Web site is filtering traffic on port 1000, probably because the same port number is used by malicious software. This page offers some insight:

The fact is, if you use non-standard port numbers, you're going to run into problems with some portion of your user base. It's up to individual network administrators to determine which ports they allow and which they block, and each of your customers probably has different administrators, and they may even have multiple administrators managing their local network, their wide-area network, and their ISP's network. My recommendation is to use Port 80, and configure HTTP host headers. Here's some more information:

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