Which operating system are you using?
Which program did you use to open .zip files before? Did you try uninstalling, then reinstalling it?
Have you checked for spyware problems.
If you're using Windows ME or XP, have you tried using System Restore?
Hope this helps and let us know more.
Whenever I try to open a .zip file now, Solitaire opens instead. What can I do to open .zip files?