How could I go about designing a computer case.

Solution
Download Google Sketchup (think a 3d modelling software for dummies), download models of the various components from the Sketchup Warehouse (plenty of people have uploaded HDD's, mobo's, graphics cards, etc) and start playing around.

Probably worth doodling on paper to start with, get some ideas. Going straight into 3D modelling, especially if your unfamiliar with the software, can be be somewhat creatively stifling so its good to already have an idea down.
Download Google Sketchup (think a 3d modelling software for dummies), download models of the various components from the Sketchup Warehouse (plenty of people have uploaded HDD's, mobo's, graphics cards, etc) and start playing around.

Probably worth doodling on paper to start with, get some ideas. Going straight into 3D modelling, especially if your unfamiliar with the software, can be be somewhat creatively stifling so its good to already have an idea down.
 
Solution