Left 4 Dead 2

Minimum
CPU: Intel Pentium 4 3 GHz
RAM: 1 GB of RAM (2 GB for Vista and 7)
VGA: NVIDIA GeForce 6600 128 MB or ATi Radeon X800 128 MB
DX: DirectX 9.0c
OS: Windows Vista, XP, 2000
HDD: 13 GB of free hard disk space
Sound: DirectX 9.0c compatible sound card

Recommended
CPU: Intel Core 2 Duo E4600
RAM: 1 GB of RAM (2 GB for Vista and 7)
VGA: NVIDIA GeForce 7600 GS 128 MB or ATi Radeon X1600 128 MB
DX: DirectX 9.0c
OS: Windows Vista, XP, 2000
HDD: 13 GB of free hard disk space
Sound: DirectX 9.0c compatible sound card

Download Link

Google Driver = Download Lift 4 Dead 2
MediaFire = Full Download

Left 4 Dead

CPU:  3.0GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
RAM: 1 GB for XP / 2 GB for Vista
VGA:  DirectX 9 compatible video card (Video card must be 256 MB or more and should be a DirectX 9 compatible with support for pixel shader 2.0)
DX: DirectX 9.0c
OS: Windows Vista, XP, 2000
Sound: DirectX 9.0c compatible sound card
HDD:  At least 7.5 GB of free space

Download Link

Google Driver = Download Left 4 Dead MediaFire = Part1, Part2, Part3, Part4, Part5, Part6, Part7, Part8, Part9, Part10, Part11, update