Word 2003 did not include forward compatibility with Word 2007, so that would be proper.
However the good news is that Microsoft has a Compatibility Pack for Office 2003 users to enable them to open, edit, and save documents in the new Office 2007 formats.
http://office.microsoft.com/en-us/products/HA101686761033.aspx
It would save the expense of purchasing the full Office 2007 product.
I hope that helps.
Mark
> I am not able to open word documents ( office 2007) in a computer that has office 2003 installed?
> Can i install both office 2003 and 2007 in a single PC?

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