A pk3 in reality is just a zip file so some archive utilities will show the same icon as an actual .zip file. Turn on file extensions so you can see what the file actually is instead of guessing based on the icon.
http://www.granneman.com/techinfo/windo ... xtensions/Built into every version of Windows since XP, if you right click on a zip archive, one of the first options will be Extract All. This will probably not appear for a pk3.
Unless you have an actual reason to yews anything else (I've heard winrar has some specific uses), you should be using this for archive purposes.
http://www.7-zip.org/Once that's installed, you can right click archives (.zip etc), go to 7-Zip, and choose Extract Here.
Once you have a .pk3 and not a .zip, the pk3 goes in base. Do not put a .zip in base, and do not put the *contents* of a .pk3 in base. You should see at a minimum, assets0.pk3, assets1.pk3, etc already in this directory.