Minimum requirements for L4D:
Operating System: Windows Vista, XP or 2000
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 Card: Direct X 9 compatible (must be 256 MB or more with support for pixel shader 2.0)
Sound Card: Direct X9.0c compatible
I thing medium settings will run perfectly. High setting can lag.
Wait, so it won't work for Windows 7?