Height of paranoia

It's a given that all workstations have XP firewall enabled, an enterprise grade antivirus and Windows defender installed. I am the security guy.

The need is that there are a couple top management executives that have highly confidential data/emails residing on their desktops, and quite a few times, the information seems to have leaked out.

Discounting the 'word of mouth' of their secretaries or the end recipients of that information, I want to take as many precautions from the IT security perspective as possible and even bring our domain admins and helpdesk personnel into the realm of doubt.

We have a Windows 20003/exchange 2003 environment of about a 2000 users. Here's what I have thought:

1. If I detach these executive PCs from the domain. Mails will stop landing in MS Outlook. Is there a way around? Also DNS security doesn't register any PC unless it's joined to a domain. I thought of this to make it out of bounds by system/domain admins. I have a feeling that their port 3389 gets accessed when they aren't around.

2. Alternatively, create a private vlan on the core switch and make these PCs as it's members. Put an ACL and deny everything except ports required to authenticate to AD and exchange and few other web applications. Monitor port memberships regularly.

3. How to secure their emails from exchange admins (it's the height, I know).

Pls advise!!