Weird problem: IE8 user can't authenticate with web service

I have an asp.net app. It has a page that requires authentication. The authenticated user can view the page because he/she is authenticated. The page makes a jQuery Ajax call to a WCF service. The WCF service checks that the user is authenticated via HttpContext. I have a user that is using WinXP and IE8. This user can authenticate to the page, but when the Ajax call is made from the page to the wb service, the user recieves my "session not authenticated" message on the page, generated by the service and displayed on the page. When I use the same OS/browser combo, the page and service work just fine, as expected; no errors.

What option in this user's IE settings would cause this beha开发者_运维问答vior?

It turns out that I was just being stupid and was violating the "same origin principle". My service was at mydomain.com and my user was probably on a page at www.mydomain.com. The service would fail because the domains didn't match completely. So, I setup some re-write rules on the server so that no matter what variant of the domain that was requested by the browser, it would always re-write to www.mydomain.com. Then, I simply set the the service call to the that domain (or in this case, just used window.location.hostname), and all problems were resolved. Big "D'oh" moment.