Uplink

Uplink is a hacking game published by Ambrosia Software. You must hack into other peoples computers and do anything from change social security information, to stealing files from a server. You can later upgrade your computers CPU, buy new programs to help your hacks, and more. It's a very stressful game because if you lose your reputation in the virtual hacking world, no one will hire you and you will have to restart your progress.

You may buy Uplink from its website: Uplink
If that's not your style, you can find free serial numbers on the internet...