I have Office 2003, 2007 and 2010 on mine (I decided to remove Office 97 and Office XP but they probably would still be OK). The secret is to make sure that they are installed into different folders/directories.
You can select/deselect some of the Office programs when you do a 'Custom' install.
PowerPivot files cannot be opened in Excel 2007 or earlier.
That workbook can be opened in Excel 2007, but only as a static snapshot; you can't edit or otherwise interact with the underlying data, since only Excel 2010 has the under-the-hood wizardry needed to talk to PowerPivot.