We can't really help you without your specs, but here are the requirements for Portal 2
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OS: Windows 7 / Vista / XP
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Processor: 3.0 GHz P4, Dual Core 2.0 (or higher) or AMD64X2 (or higher)
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Memory: 1GB XP / 2GB Vista
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Hard Disk Space: At least 7.6 GB of Space
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Video: Video card must be 128 MB or more and should be a DirectX 9-compatible with support for Pixel Shader 2.0b (ATI Radeon X800 or higher / NVIDIA GeForce 7600 or higher / Intel HD Graphics 2000 or higher).
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Audio: DirectX 9.0c compatible