Counterstrike 1.6 and you can play online, no crack no serial, no patch, nothing needed. Just install and you are done. If you look, you have a couple thouzand servers where you can play.
Play against the CPU
Play against your friends with LAN
Most pc or laptops easly run this game:
Minimum 64 Mb videoCard
CPU speed: minimum: 900 MHZ
If you have a middle class pc, you can run it easly