XBAP crashing Internet Explorer

Hi,

On one of my machines (controlled through group policy so I can't see
whether XBAP are allowed in Internet Properties | Security | Custom Level),
whenever I try to navigate to an XBAP (whether on-line or from my local hard
disk) Internet Explorer 7 will show "Connecting..." in a tab, and
intermittently display a dialog (which vanishes very quickly), but never
actually start the XBAP.

Why is this happening? BTW, I am able to debug XBAPs via Visual Studio 2005
(which also loads Internet Explorer) so I am a little confused.

Any pointers?

Joubert