|You can simply install the older version of Office.|
Both versions can co-exist peacefully (with a few nagging irritations.)
Except for Outlook.
You can have only one version of Outlook.
I know Microsoft claims you have to install the earlier version first.
That is not true.
You can install the newest version first, and then the earlier version.
I run both Office 2003 and 2007 at home.
I find it easier to open Office 2007 files in Office 2007, and 2003 files in 2003.