There are 1% chance, u can find a serial that no one had ever used before, unless u bought ur cs. if u want cracked version of CS use>> AAAA-AAAA-AAAA-AAAA (it works) allows u to play in some servers, ex. Like this server. but for some, when u try to connect, it shows up as invalid CD keys.
If u wanna play in every server, still recommend u to buy ur CS.