Thanks Paul, I don't feel quite so lonely now!!
"supports partial rending" is definitely switch ON.
Not sure about the OS, but there are other applications that run on this server that use AJAX perfectly OK, inclucing one that is hosted inside an Iframe within this DNN Portal!!!!!!
I have read elsewhere that PageBlaster has caused AJAX to stop working. Now I don't use PageBlaster, but are there other modules that also cause AJAX to fail?
Anyone out there know or have any other ideas?
PS. This thread is now showing 7 posts(before this one is put up!) so there is something very wrong !!