Virii that are never executed are generally harmless. Once the virus is executed, it loads into memory and does whatever it wants. Without being executed, it is just another file sitting on your harddrive.
Of course, if the virus is saved in a location (like your "Startup" folder) that will cause it to run on the next reboot or during some common system task or user task, it will be able to load itself into memory at that point. You don't really have to worry about this happening with virii you get in your email, etc.