[Marc P.]

When winzip is installed, one of the options during the installation is called "Winzip Shell integration"- it's handy because it adds Winzip functionality to right clicks and file associations, but if you want to get back to Compressed folders, it may be easier just to delete a registry key. All the standard warnings apply to editing the registry- if you delete the wrong entry or change something else, bad things can happen.

I have winzip installed here and just tested this. Click start, run, type in "regedit".

Under HKEY_CLASSES_ROOT, look for .zip, right click it export it first (to back it up in case something goes wrong), and then delete it. Close the registry editor, and the next time you double click a zip file, it will regenerate the .zip class in the registry with the default Compressed folders assocations.