Half-Life 2 is available on Steam. To install, simply open your Steam client, navigate to your Library, find Half-Life 2, and click 'Install'. Ensure you have sufficient disk space (approximately 7 GB). For optimal performance, Valve recommends a 1.7 GHz Processor, 512MB RAM, DirectX 8.1 level Graphics Card (SSE compatible), and Windows 2000/XP/ME/98. While these are minimums, a modern system will run the game flawlessly at higher resolutions and frame rates. Always ensure your graphics drivers are up to date for the best experience.

Installation & System Requirements — Half-Life 2 Guide