If you are running XP and do not have winzip or winrar etc loaded, its
part of the OS and simply double clicking the file will open it. Xp
treats the file just like a folder. And basically it is, its a group
of files in one container, isn't that what a folder is.
Either way the right-click menu on that file should have a choice to
extract the files in the zip.
....if not:
If you don't have winzip, you open (double click) the file and copy the
contents to another folder (thus extracting them)
If you have winzip or winrar then double click and open the file, there
will be an extract in the toolbar to allow you to extract all or
selected files to another folder.