1. From the taskbar, click on the Start button
2. In the field directly above Start type: REGEDIT
3. Press the Enter key on the keyboard
4. Click Continue in the User Account Control dialog
5. Browse to:
   HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.zip
   (HKCU is HKEY_CURRENT_USER) – To do this:
   • Click the triangle next to each item (HKCU, then Software, etc)
   • When you get to .zip click the triangle next to it also
     
6. Click UserChoice to select it
7. Press the Delete key to delete UserChoice
   
8. Close the Registry Editor (REGEDIT)

If Zip files are not associated with WinZip after completing the steps above:

1. Right click the WinZip shortcut on the desktop
2. Click Run as administrator
3. If a dialog displays offering to associate WinZip with Zip files, click Yes
4. If the dialog above did not display, open the Configuration Dialog
• Winzip ribbon interface: Click Options in the Settings tab
• Legacy menus/toolbar: Choose Configuration on the Options menu
5. Click the System tab and click the Associations button
6. Scroll to and check ZIP
7. Click OK both in the Associations dialog and the Configuration dialog

http://kb.winzip.com/kb/entry/155/