You can download it from here
Call of Duty 4 Demo - Call of Duty 4 - Yahoo! Games
Recommended system specifications are as follows:
Required (minimum) Specs
CPU: Intel(R) Pentium(R) 4 2.4 GHz or AMD(R) Athlon(TM) 64 2800+ processor or any 1.8Ghz Dual Core Processor or better supported
RAM: 512MB RAM (768MB for Windows Vista)
Hard Drive: 8GB of free space
Video card: NVIDIA(R) Geforce(TM) 6600 or better or ATI(R) Radeon(R) 9800 Pro or better
Recommended Specs
CPU: 2.4 GHz dual core or better
RAM: 1G for XP; 2G for Vista
Hard Drive: 8GB of free space
Video card: 3.0 Shader Support recommended. Nvidia Geforce 7800 or better or ATI Radeon X1800 or better