What would cause an IE 8 user to not be able to execute jQuery getJSON calls?

A test user of my site (running jQuery 1.开发者_如何学Go4.4) is not able to execute any of the JSONP jQuery getJSON calls.

He's using the Internet Explorer 8 on Windows XP.

Any ideas what would cause this or how to work around it?

This question is a bit vague for me to be of any more help than to point you in this direction:

Internet Explorer VPC Images

Using the VPC image you will be able to test in XP / IE8 if you think this is an OS / Browser specific issue.