This is an answer a year (!) late, but I had this exact problem (left shift key, right?) and solved it just now.
For the people who have the problem of a specific key causing a windows shutdown: if it is an external keyboard, hold the problem key down when you re-plug in your keyboard. If this is a laptop keyboard, press the shift key down until your initial boot up process is complete. It seems to be caused by the computer confusing a depressed shift key signal with Alt + F4.
If you are not sure which key exactly triggers the shutdown, you can download Shutdownguard and start pressing keys one by one to see which one triggers the shutdown process.