if (window.external) req = window.external.XMLHttpRequest;
The problem was XMLHttpRequest was not a property of window.external and was throwing an exception and so our AJAX callback was not working. This all came due to a Windows Update earlier this week on my Vista computer. My first thought was this was going to break customers web applications who are using version 2.5 and higher of Spread for ASP.NET, but the development team said this line of code was added only to version 4 for use with our Spread for SharePoint product.
SO... We have fixed the line of code and have posted the new htc file on our ftp server.
AND... We should be updating the latest maintenance release setups with the new client script file shortly.
If you are using Spread for ASP.NET 4 and have users browsing to it on IE7 in Vista, then you will need to get this latest script file and replace it in your web application.