can't play or edit .mpg files

Help! I’ve had multiple problems trying to do this before, and I’m really determined to fix it now. I recorded a video earlier on my sony handycam, but now I can’t play, download, or edit any of the files! I’ve tried plugging the camera into my XP, vista, AND my mac, but none of them work. I’m currently using my mac because I wanted to edit the video using iMovie. quicktime won’t play the video, and I’ve tried to use several free converters to convert to .mov, none of which have worked. ideas?

When you plug the camera in XP or Vista, are you able to access the individual files? And if so are you able to copy them via Windows Explorer (not using the bundled software).
If you’re able to do that, can you feed the videos into a diagnostic utility like one of the ones listed here http://www.moviecodec.com/topics/61931p1.html and report what it says.