a guess: if you work on the files on the mac, make sure to save them (under "save as...", for example) as a windows version of powerpoint. the mac will still be able to open the files then, and this should fix the windows problem.
the mac problem might be that it's trying to save a resource fork to the windows server and failing, so the file is unopenable by the mac after a seemingly successful save.. saving as a windows powerpoint file should also fix this, although I'm not sure why this problem would happen anymore, to be honest (then again, i haven't worked with these kinds of files for a while or NT servers.. ever).