COD4 Online is fere for PC regardless of your internet connection.
Yes you can, usually PC game work on the PC or Mac, maybe rarely only the PC but for cod4 they most likely made for both computer types.
CoD4
check www.callofduty.com
figure out the buttons then get used to them
Download a torrent client and go find it.
It only works on PC. And you simply download a mod off the Internet, multiple sites have them. Once in the CoD 5 main menu on PC simply select mods once downloaded and select launch.
you buy the game or you download it to get the key
try upgrade it http://www.gamershell.com/download_28304.shtml
you cannot hack weapons on any console
You only have to pay for Xbox 360 online. You don't need to pay a monthly fee for PS3 or PC.
this really isn't an apropriate question, but try www.warez-bb.org