On Windows Vista you can normally open a .zip file by right-clicking on the file and using Extract All... from the popup menu.
However, if you have assigned the .zip file type to a program (possibly by accident) then Vista will no longer show the Extract All... option and you will not be able to unzip the file without using a third party program.
1. In Windows, choose Start -> Run -> regedit (Type regedit in the box and press enter)
2. Then look for..
HKEY_CURRENT_USER
-> Software
--> Microsoft
---> Windows
----> CurrentVersion
-----> Explorer
------> FileExts
-------> .zip
--------> UserChoice
3. Select this key and delete it (use the delete key or Edit --> Delete).
4. Then choose File --> Exit
Important: If you are not familiar with this area of Windows, be very careful because making changes to the registry can prevent Windiows from running correctly. If you are not confident then best to ask someone who is familiar with it to assist.
KnowledgeBase Article: Extract All option in Vista is missing