I have Directx in my program files but Myst V won't run and when I go to the detection tool it says that it is not on my pc. When I try to download it from the disc, a box appears saying that a cabinet file necessary for installation cannot be trusted.

Download and install the latest end-user runtime release of DirectX 9.0c from Microsoft. It's available here.

Be certain to log into Windows with full Administrator privileges and disable your anti-virus software, before installing DirectX. Reboot when installation is complete.

If you also need to change your video driver, see Myst V: Installing Video Drivers With Win XP, one of the stickied topics at the top of this Forum.