Internet Explorer z-index problem with lightbox popup - can jQuery fix it?

Hi folks,

I am working on a site and seem to be having a major issue with Internet Explorer and z-index.

If you visit this website and click the "Rewards Store" down the left hand column, it has a lightbox popup that creates an overlay but I cannot get the overlay to go behind the login box popup.

www.allfix.co.uk

Any assistance would be much appreciated!