directX problems

alright i'm having lots of problems with direct x. in call of duty 4, game just gives me black screen, freezes and bye bye game. but anyways, i try to fix it, and everything i tried failed, now all i have that is all i think the problem is, i need to reinstall direct x 9c but when i try to install this is what it gives me:

A cabinet file necessary for installation cannot be trusted. Please verify the Cryptographic Services are enabled and the cabinet file certificate is valid.

yes cryptographic services is working fine, i checked, it is set to automatic start and it is running. i just want to know what the hell am i suppose to do to fix this cause I'm really getting annoyed by that.

if specs would even help here they are:

-Intel Pentium d 3.0GHz (overclocked to 3.2, and no, even when i take off overclock it still gives me all the problems)
-antazone as-c1000 CPU cooler (CPU runs usually at around 25 degrees Celsius)
-ECS PA1 MVP 2.0 motherboard
-corsair TWINX 1GB ram DDR2 667MHz(512 x2)
-ATI Radeon 2600 HD PRO 512MB PCI-E
-ultra x-finity 600W power supply
-2 seagate barracuda 250GB sata 3GB hard drives
-samsung writemaster dvd burner

What exactly have u done it
have you reinstalled cod4
updated graphic card drivers
Do you have anyprograms running in the background all the programs by the clock
It could be overheating but i would think unlikly does this happen to anyother game you have or is cod 4 the most highest spec game in ur collection