Aaaah, good ol' M$ apps!!
Okay, first things first. Did you say the PC you created the files with doesn't recognize the format now? Or was it amother PC?
If it was the same PC, remove PP from Add & Remove then reboot, shut off the Network connection and ALL AV or spyware scanners, along with everything else that isn't needed for the PC to run (IM, firewall, Weatherbug, you name it).
Reinstall Power Point. Reboot.
Try opening the files again, then try creating a small file and try opening that if the first file chokes.
Either the .dll's went to the wrong addresses, or the registry entries didn't 'take'. A reinstall should help.
That, or the CD needs cleaned/replaced!