Hardware Requirements

* Supported OS: Microsoft Windows XP / Vista / Vista64
* Processor: Pentium 4 3.0GHz
* Memory: 1 GB
* Graphics: 128 MB, Shader model 2.0, ATI 9600 or better

Recommended:

* Supported OS: Microsoft Windows XP / Vista / Vista64
* Processor: Intel core 2 duo 2.4GHz
* Memory: 1 GB
* Graphics: 128 MB, Shader model 3.0, NVidia 7600 or better

MORE INFO:
http://pc.gamezone.com/gamesell/p31202.htm
http://www.gamespot.com/pc/action/left4dead/index.html?tag=result;title;1
http://uk.pc.ign.com/objects/818/818215.html

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Requirements
* OS: Windows XP, Vista, Vista64
* CPU: 3.0 GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
* RAM: 1 GB for XP / 2GB for Vista
* Disc drive: DVD-ROM Drive
* Hard drive: At least 7.5 GB of free space
* Video: Direct X 9 compatible video card -- Video card must be 128 MB or more and should be a DirectX 9-compatible with support for pixel shader 2.0 (ATI 9600 or high. Or, Nvidia 6600 or higher)
* Sound: Direct X9.0c compatible sound card