I think as far as support for the legacy binary MS Office formats go, OpenOffice.org is your best bet. Support for creating those files is usually much better than for reading, so most compatibility issues you will encounter are usually on the "read a file created with MS Office" side, instead of "read a file in MS Office which was created by OpenOffice". Usually implementations aren't dead wrong but rather incomplete, so opening files might cause them to lose some features or fidelity, but creating usually is painless.
Other office suites you may want to try (although they don't have the extensive history and struggle for MS Office support as OOo) are KOffice and GNOME Office.
Also note that the binary file format specifications are all made public under Microsoft's Open Specification Promise so they're not entirely1 closed for quite a while anymore. Support will likely get better.
1 There are still some parts that require either reverse-engineering of previous Office versions or some guesswork2, but for the most part the specifications are complete and publicly accessible.
2 I also don't think most people need the compatibility "features", such as "line spacing as in Word 5.5" or similar so that's essentially a moot point for most practical implementations.
The answer is structured to convert all documents, not just Word documents.
Say you got tons of Excel sheets, PowerPoint presentations and Word documents on your computer that were written in Office XP or 2003. How do you convert all these files to the new Office 2007 format.
One option is that you open all of them in the associated Office program and manually save them in the newer (docx, xlsx or pptx) format. Or follow these steps and convert all documents in one go.
Step 1: Download Migration Manager kit and extract it into a new folder - say: c:\office.
office-pack
Step 2: Download and install the Office Pack - this step is required even if you have Microsoft Office 2007 already installed on your computer.
Step 3: Assuming that you extracted the Office Manager files in c:\office directory, go c:\office\tools, open ofc.ini using notepad and add the following line.
fldr=c:\users\labnol\documents
This refers to the folder location that holds your office files. I am pointing this to my Documents folder but it could be different on your machine.
Step 4: Open command prompt and go to c:\office\tools. There you’ll see a utility called ofc.exe - this is the Office File Converter that will convert all old Office files to the new 2007 Office document format in bulk. Run.
convert-docs
Immediately all old Office files in that folder (and sub-folders) will get converted to the new format and are saved in a new folder.
This utility works with Word (doc to docx), Access databases, PowerPoint (ppt to pptx), Visio diagrams, Excel (xls to xlsx) and Microsoft Project files. The conversion may however fail for password protected documents.
Taken from here.
Best Answer
You are going to find this a difficult task to accomplish, and even more so if it's on an ongoing basis. The problem is that you will have to convert to the least common denominator. If the oldest version of Powerpoint installed, for example, is Powerpoint 2000 then all files must be converted backwards to the 2000 version. While this is possible, it will mean that any of the newer features will get dropped from the file if they are being used in the presentation. And it will likely cause formatting and other problems in the file.
If the oldest version is 2003, it's a little easier. There are both software utilities such as those offered by Batchworks and online services such as Zamzar which offer conversion possibilities, but none of the software or services I've found offer good compatibility with any version older than 2003.
Your best solution is to convince your client that they need to close the gap between versions in use, but I know that's often a harder sell than trying to explain all the hours of productivity they are losing by not doing so.