Force Single Instance Of Excel When Opening New Workbook From Email?

I have multiple buyers that use an Excel workbook containing several macros to perform edits on other workbooks they receive via email. Generally, they have the workbook containing the macros open, and they can double-click the workbooks they receive in the email messages to open them, and the macros are available from the "master" workbook.

Recently, however, some of the buyers have been getting new computers running Windows 8 and Office 10. Now when they have the "master" workbook open, and double-click the attached workbook in an email, (Outlook 10), the new workbook opens in a new instance of Excel, and as a result the macros are not available to run on the newly opened workbook.

The work around has been to right-click the attachment and save it to their desktop, and then use the File/Open command from the "master" to open the new file. This is a hassle and takes extra time.

I'm sure I am overlooking a setting somewhere that tells Excel to always open new workbooks using the same instance of Excel, (there are two people using the new systems that aren't having problems, and two that are), but for the life of me I can't find where to set these parameters.