If you dont have winzip heres an easy way to get around having to use winzip to open .pk3 files ( but it might only work with windows XP )
 
 1st: Right-Click on start and click "Explore All Users".
 2nd: Click on "Tools" ( at the top ) then "Folder Options..."
 3rd: Click on "File Types" ( and wait for it to load )
 4th: Click on "New"
 5th: Then where it says "File Extension" type: pk3
 6th: Then click on "Advanced" 
 7th: Next to where it says Associated File Type there is a drop down click the drop down arrow and find "Compressed (zipped) Folder"
 8th: Click on "OK" then "OK" again
 
 And Voila! You can now double click and open .pk3s!
 But beware change anything in side of them and your game will totally mess up you wont be able to join any servers. So just copy what you want to change and put them in another .pk3 (not the assets!) By doing this you
 
 1st: Right click in side of your base ( not on a file though! ) and put your mouse over "New" then click "New Compressed ( zipped ) Folder" 
 2nd: rename the "New Compressed ( zipped ) Folder.Zip" to: ( The Name Of Your Mod ).pk3
 3rd: Press "Enter"
 4th: When something pops up click "Yes"
 
 And there you have it you own .pk3 file now just put your changes in there and you have your very own mod!!!
  
 
  
  
    Heh, this is the easiest 
 
 Simply rename the .pk3 to a .zip :D
  
 
  
  
    I have a better idea, instead of fooling aorund with all that bologna, and winzip, just get winrar, its ten times better :) Then just associate .pk3 extension to winrar
  
 
  
  
    Originally posted by acdcfanbill 
 I have a better idea, instead of fooling aorund with all that bologna, and winzip, just get winrar, its ten times better :) Then just associate .pk3 extension to winrar 
 
 winRAR is at least a hundred times better (rarlabs.com) it's only a little bit slower than zip, it pwns it in everything else