For Office 2010 Version which will offer an upgrade or an install option to overwrite files in 2007 version or any earlier version of the product like Office 2003 or Office XP with Office 2010.
here are the steps on How to Install two Microsoft Office Product Without Uninstalling the other and sub application Category Run Simultaneously such as Word, Excel, PowerPoint, Publisher, OneNote, SharePoint etc
1.Run Office 2010 installation wizard. Select Customize when prompted to choose the preferred type of Office 2010 installation of either “Upgrade” or “Customize”.
2.Click Browse and select another installation folder path for Office 2010 which is not the same with existing Office installation path. For example, C:\Program Files\Microsoft Office 2010\.
3.The setup will install Office 2010 and retain Office 2007 and any other older versions of Office apps.
and the default files for documents spreadsheet and other files will be the newer version however you can change the default file opening via control panel
note:Older Outlook Files will be uninstalled by this method and only 2010 version Outlook version Will work by this method