It might have been a missing font but that shouldn't keep the file from opening. It could also be a missing picture or video. Try exporting from keynote again and see if it gives you any document warnings. if you get any warnings then post them here and i can try to help some more.
Are you able to upgrate to 10.9? Did you buy keynote from the app store? If so, you could try the upgrades and see if it's better. When I upgraded today it put the older version of keynote into an iWork 09 folder so you can probably go back if it doesn't work and you don't like the newer keynote.
So you must have bought it on CD then?
Did you try exporting again? keynote is usually pretty good about warning you if there will be changes made for PP compatibility. If you see any warnings and can post them in this forum, they might help someone here figure out what happened.
I have one other idea. Did your PC say that the file was missing an extension? If so, you might just need to "rename" the file and add ".ppt" to the end of it. That would tell the PC to open the file using powerpoint. Without it, the pc probably doesn't know what kind of file it is, and may give you an error message.