I would guess that if you have Windows 7 Ultimate you should be able to run the Sage program in XP mode in windows 7. I had same problem with another brand of ERP/CRM software and it runs perfect using the XP mode under Windows 7.
The Windows XP Mode will be what you want to use. This is a Windows XP virtual machine which runs on the users Windows 7 desktop. You install MAS 500 on the Windows XP virtual machine instead of the Windows 7 parent OS. The shortcut will be available on the Windows 7 Start Menu. When the application launches the Windows XP OS is woken up and the MAS 500 is started. The only different that the user will see is that the title bar will look a little different as it will have the Windows XP title bar instead of the Windows 7 title bar.
You will need to manage the patching of the Windows XP OS as well as the Windows 7 OS.