Vista has zip support built-in. When you changed the default program, you changed your ".zip" file association and lost the link to the default actions. To restore the default zip association you can do this:
Click on Start, All Programs, Accessories.
Locate the "Command Prompt" entry under accessories and right-click it. Choose Run as administrator.
Reply "yes" when asked if you are sure that you want to open the command prompt as an administrator.
When the command prompt comes up, type:
assoc .zip=CompressedFolder
and then press Enter.