Opening Powerpoint files in Mac and PC

Hi. I sometimes have to create presentations on my MacBook using Powerpoint (I know! Keynote is better and I love it.). I am using Powerpoint 2004 for Mac version 11. I recently discovered that I can't open some powerpoint files on Windows-based computers with Microsoft Powerpoint 2003 Professional Edition, but could open them with a 2002 version. Does this have something to do with the windows version or my Mac? Since I have to share the files with my colleagues, I often have to open the files from someone else's computer (which has a 2002 PPT version) then save the file to a PPT 97-2003 & 95 version just so my colleagues with a 2003 version can open the files. Is there a simpler way to solve this compatibility problem? Hope someone can help me. Thanks!