Are you able to drag-and-drop your files into their folders for playback or does the Zen freeze just like it does when syncing?
If you want to know if the problem is the Zen or with the computer you can try either installing the Zen on another computer or returning your Zen for another; or both. Then you should see how/if the problem carries over to the new Zen or not. If you still have the same issue with a new Zen, then I would say their is more to your computer/setup causing the issue. If the issue goes away, you will know it was the Zen. Since you are saying it locks up your computer too, I am leaning towards more of an issue with your computer more so then the Zen.
If everything you do, no matter what the media you are trying to copy/sync is "freezing" the Zen and computer, then I hate to say it but it may be faster to just try the Zen on another computer to see how it reacts firsts (if you have another you can try it on). If you can't, I would just get it replaced. You may be able to get this done in less time then having everyone here try to figure out the issue. In the mean time, you may try uninstallingWMP11/rolling back to WMP10 to see if that helps. If it doesn't try uninstalling all Creative software, then reinstalling WMP11 and trying again.