I don't know what the issue is, but you might want to try activating the 'hidden' administrator account on Vista. Some of the myriad problems I've had with Vista have been rectified when I've used this account. Google the topic and you should find simple step by step instructions on how to do this.
Though I don't want to get into a Vista slanging match, I think it's hardly encouraging that Dell has resumed shipping machines with XP installed for business customers who request it.
Simon Bonner