I had this problem too when Office 2010 was installed, then downgraded to 2003. We have the older versions and only want to use those for test computers as they do not require the latest versions of Office. Here's what I did and so far it seems to be working.
Fix the file association classes by modifying the registry. You may need to modify the following (the first section removes any culprit items, this may need to be run under each user logged on a computer that has used Office before):
Windows Registry Editor Version 5.00
[-HKEY_CLASSES_ROOT\.docx]
[-HKEY_CLASSES_ROOT\.pptx]
[-HKEY_CLASSES_ROOT\.xlsx]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.docx]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.pptx]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.xlsx]
[HKEY_CLASSES_ROOT\.docx]
@="Word.Document.8"
"Content Type"="application/msword"
[HKEY_CLASSES_ROOT\.docx\PersistentHandler]
@="{98DE59A0-D175-11CD-A7BD-00006B827D94}"
[HKEY_CLASSES_ROOT\.docx\Word.Document.8]
[HKEY_CLASSES_ROOT\.docx\Word.Document.8\ShellNew]
"FileName"="winword8.doc"
[HKEY_CLASSES_ROOT\.pptx]
@="PowerPoint.Show.8"
"Content Type"="application/vnd.ms-powerpoint"
[HKEY_CLASSES_ROOT\.pptx\PersistentHandler]
@="{98DE59A0-D175-11CD-A7BD-00006B827D94}"
[HKEY_CLASSES_ROOT\.pptx\Powerpoint.Show.8]
[HKEY_CLASSES_ROOT\.pptx\Powerpoint.Show.8\ShellNew]
"FileName"="pwrpnt11.pot"
[HKEY_CLASSES_ROOT\.xlsx]
@="Excel.Sheet.8"
"Content Type"="application/vnd.ms-excel"
[HKEY_CLASSES_ROOT\.xlsx\Excel.Sheet.8]
@=""
[HKEY_CLASSES_ROOT\.xlsx\Excel.Sheet.8\ShellNew]
"FileName"="excel9.xls"
[HKEY_CLASSES_ROOT\.xlsx\PersistentHandler]
@="{98DE59A0-D175-11CD-A7BD-00006B827D94}"