Do you have the same version - including the same updates - on both systems?
Do you open the attachments in the exact same way?
AFAIK, you can't urn it off, other than by closing the attachment before
closing the message or saving the attachment to the hard drive, then
opening. If the attachment opens as read-only, you shouldn't get that error
either - double clicking from the reading pane should make it read only.

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> When I open an attachment from an e-mail and then I close the e-mail or
> scroll down to other e-mails in my inbox but keep the attachment open, I
[quoted text clipped - 6 lines]
> possible
> because my computer at home does not behave the same way. Thanks