materials and textures go missing on my machine.

When I open unity projects, some of the textures are missing. The textures are still in the project but are not on the objects used in the scene. the objects are usually purple instead. anyways I've opened the same project on other machines and it works fine. but when opening it on mine it all purple with errors that shouldn't be there.

I've also used assets from unity store and tutorial projects and they have the exact same issue.

I'm using windows 8 and all machines that I've tested my projects on have the same operating system