The file in the Trash still has the .zip extension while the one in
downloads has only the .tar.
.zip? There are no .zip files at that webpage.
What it sounds like is you've got Stuffit Expander or some similar utility configured to auto-expand compressed files, and it's messing the file up somehow. If it's a hangover from a former Leopard installation, maybe the problem is the utility isn't quite compatible with Snow Leopard.
Another possibility is your browser is doing this. Check its preferences for something like "auto-open downloaded files".
