Your statements in the last paragraph aren't very clear. by the way. You say it's a problem at user level. So if the user logs in on another PC in the network, it doesn't work either. And if another user logs on on this PC it works. That's an error in that users roaming profile.
But then you say all PC's are equal. That isn't a contradiction. In the 2 sentences before you already showed not all users are equal.

Assuming it's the PC, not the user: those PC's are not the same. If they were, they would behave the same. You only THINK they are the same.

Now for the solutions:
1. Tell the user (or all users of this PC) to not print mails from this PC from the preview pane.
2. Switch off the preview pane as a safety measure. After all, it's recommended to not use the preview pane at all, because it might auto-execute html with bad scripts, before the user had has the opportunity to delete it.
3a. If it's really the PC, not the user: back to the same image as all those other "same" PC's. Then it's really the same.
3b. If it's really the user: delete his account on all PC's; delete his roaming profile and start all over.

Kees