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.