Word 97-2003 and Word 2010 files have the .DOC and .DOCX extensions respectively by default. Their file type descriptions are also different, so naturally if you sort according to Type in Windows Explorer they'll be grouped separately.
A simple solution would be to sort according to Name or any other common characteristic.
If you want to sort by Type and still have them grouped together, you can try editing the description via regedit (registry editor).
The description for Word 2010 .DOCX files is "Microsoft Word Document" (see HKEY_CLASSES_ROOT\Word.Document.12). Leave that be.
The description for Word 97-2003 .DOC files is "Microsoft Word 97 - 2003 Document", so you can go change the (Default) value at HKEY_CLASSES_ROOT\Word.Document.8 to be the same as that for .DOCX files.
Reboot and check if both file types now have the same description and are sorted together in Explorer.
Note: This has not been tested by me so do it at your own risk and do not complain if Word stops functioning properly, or resets the registry entries on its own.