Guess the lad means he downloaded some pk3 files in a zip file, extracted them from the zipfile and wants to check out what is inside the pk3.
Well, pk3 is also a zip extension, like .zip and .rar. So basically inside the .pk3 file will be files like skins, models etc. To get to them, simply open the .pk3 with e.g. winrar.
If you doubleclick on a .pk3, windows will say it does not know any program associated with it and asks you to locate a program. Just point out to your winrar executable and voila.....
I DUnno if this works with winzip also, but good changes it will (and otherwise... download winrar