In order for you to play Portal 2 on Windows with minimal graphics settings, your computer must meet the following requirements
Windows 7 / Vista / XP
3.0 GHz P4, Dual Core 2.0 (or higher) or AMD64X2 (or higher)
2 GB RAM
Video card must be 128 MB or more and with support for Pixel Shader 2.0b (ATI Radeon X800 or higher / NVIDIA GeForce 7600 or higher / Intel HD Graphics 2000 or higher).
8 GB available space
DirectX 9.0c compatible