As per Microsoft, you need to install the oldest version first and the newest version last, if you intend to use multiple versions of MS Office on the same machine. There are KB articles for each version of Office that explain this, http://support.microsoft.com/kb/2121447 is for Office 2010 and it has the KB #s for other supported versions of Office as well. But there is a big assumption in these articles, that all of us would like to make the latest version as the default version, this is not always true and hence the problem. Like in my scenario, there was a need to install office 2010 on a server that already had office 2003 and the 2003 version needed to be made the default office application. So, if I install Office 2010 on top of 2003, then the newer one would become the default version which is NOT what is needed, so the workaround was to repair the office 2003 version AFTER the installation of the 2010 version so that it becomes the default one.
This is not a one time affair though, Microsoft recommends following the same order (oldest one first and newest one last) while installing patches to the office versions as well. Now, what happens if the only patches released in a given month apply to office 2010, after you apply those patches, the 2010 version will become the default version. So, we again need to follow the same workaround i.e repair the office 2003 version to make it the default app.
I have only tested this for Excel but I assume this should work on the others, such as word and PowerPoint as well. Also, let me clarify that here, I’m talking about “Installing” the office versions and not using the ‘Click-to-Run’ technology as discussed at http://support.microsoft.com/kb/982434
This is one of the reasons why it’s not such a good idea to install multiple versions of the same app on the same machine, Microsoft does not recommend installing two or more versions of MS Office on the same machine and they don’t even support the same if it has been done on a terminal server. So, the best way out is to have just one good version but if you can’t do with that then the above workaround is there for you.