I think your PC has issues on system requirements. I am sending you details about the requirements of the game.
The minimum requiremets for Call of Duty 4 are:
For OS is Win XP 32, CPU: Pentium 4 2.4GHz / Athlon 64 2800+, RAM:512 MB, GPU:GeForce 6600 / Radeon 9800 Pro, HDD:8 GB
And the Recommended system requirements for OS: Win XP 32, CPU: Core 2 Duo E4600 2.4GHz / Athlon 64 X2 Dual Core 4200+, RAM:2 GB, GPU:GeForce 7800 GT / Radeon X1800 Series 256MB, HDD:8 GB.