Windows 2000 (with Service pack 1 or higher) or Windows XP (Home/Professional/Media Edition) with Service Pack 2, or later Windows versions.
Processor
1.9GHz Intel Pentium 4 or AMD Athlon processor or equivalent
Memory
512MB RAM (1.0GB RAM is highly recommended)
Graphics
128MB Video Card with Hardware T&L and support for Pixelshader 2.0 or better
DirectX Version
DirectX9.0c
Hard Drive
At least 1GB of free hard drive space to accommodate game files and saved games. This does not include any space used by the Windows swap file.
Sound
DirectX7 compatible sound card
Go to: http://store.steampowered.com/ and download STEAM client
Click "Install Steam" (from the upper right corner)
Install and start application, login with your Account name and Password (create one if you don't have).
Please follow these instructions to activate a new retail purchase on Steam:
Launch Steam and log into your Steam account.
Click the Games Menu.
Choose Activate a Product on Steam...
Follow the onscreen instructions to complete the process.
After successful code verification go to the "MY GAMES" tab and start downloading.