http://www.codeproject.com/KB/ajax/demoajaxapp.aspx?msg=3855224
windows server 2000 IIS 5 , works with fix you detailed below 
and when uput on to a server running windows server 2003 IIS 6, with the deltails described below, the code does not work?
Yes, You need to add the extensions .ajax and ajaxj in your virtual directory configuration. Find the steps below to configure these extensions.
1. Open the IIS.
2. Open a property window of your virtual directory.
3. Click on "Configuration" button to open "Application Configuration" window under "Virtual Directory" tab
4. Under Mapping tab, click on "Add" button, it will open "Add/Edit Application Extension Mapping" window.
5. Write the extension ".ajax" in Extension textbox.
6. Click on "Browse" button, the default directory will be Framework Version directory. if it is not then select your framework version directory. it will be at "C:/WINDOWS/Microsoft.NET/Framework/vx.x.xxxxx".
7. Select "aspnet_isapi.dll" from that directory" and click on "Open" button of dialog box.
8. In Verbs frame select "Limit to" radio button and write "POST" in the textbox.
9. Uncheck the "Check that file exists" checkbox.
10. Click on "OK".
11. Same steps need to follow to add ".ajaxj" extension but in verb frame write "GET" in place of "POST" for that.