A tool to bookmark
In this case, Zamzar, a British-based online file conversion service.
In both free and paid versions, this service will convert media or document files among various formats, including some pretty odd ones.
I haven't tried it, but it lists the ability to convert the new Microsoft Office "x" formats, which are not backward compatible, to the older formats (in other words from .docx to .doc). That's a heck of a lot more convenient than downloading Microsoft's converter.
It will also convert one type of compressed file to another.
The major limitation of the free version is the relatively small total file size (100 mb, which isn't a lot if you are working with media files) and no online management of your files.