I'm glad to see that my Ajax.NET Professional  library (version 126.96.36.199 or higher) is working with Internet Explorer 5.0 and 5.5. Both versions are supporting requests using the ActiveX XmlHttpRequest object or (if ActiveXs are disabled) the IFrame proxy. I've started logging Ajax.NET requests on my demo site http://www.ajaxpro.info/ , and here is the statistic I got after collecting data for some days only (sorted by web browser agent):
Firefox 0.8 Firefox 0.9.x Firefox 1.0.x Firefox 1.4 Firefox 1.5.x Firefox 2.0a1 Konqueror 5 Microsoft Internet Explorer 5.01 Microsoft Internet Explorer 5.5 Microsoft Internet Explorer 6 Microsoft Internet Explorer 7 Netscape 7.x Netscape 8.x Opera 8.5x Opera 9 Safari (Gecko) [...] 
Support for Internet Explorer 4 I do not want to add because of the missing try..catch statements and other nice features. As my logfile is telling me there are no visits with such old web browsers. Did you every tried other AJAX frameworks with old web browsers? Feedback welcome in the Google group  post here .
Update: Because the ASP.NET browser class doesn't support all current web browsers I put a list of all user agents online at http://www.ajaxpro.info/supported_browsers.txt . I will update this page from time to time.