You can download counterstrike if you have the original halflife. Otherwise you have to buy counterstrike from a retail store. Or I think you can purchase it straight from steam and download it. Counter-strike is a mod of halflife, so you need halflife to play it, unless of course like I stated above, you buy the retail version alone from a store.
Counter-Strike Condition Zero is just awesome...
Though the single player gets a little bugging when you have to save the hostages but the other cts just go and kill off all the terrorists... had a hard time accomplishing the objective but totally loved the in-built bots package...
Just look for Steam download. You dont need the game, actually you will need the game for the cd-key, to create an account on steam, but look for steam, that is free. Around a 400mb Download.