Have you installed anything new or updated the drivers for your video card lately? This type of problem usually points towards something like this where you see a bad install or driver conflict putting a damper on things. Since this occurs with MyComputer where Windows Explorer is also tied in a cross link to system files or something was corrupted.
If you look in the general tab you will note the last option on the bottom is for single click or double clicking on a selected item. Make sure that's set for double clicking. You will note the "Restore Defaults" button seen in the screen there is another thing that should return things to normal if there are no other problems found.