Game Crash

2 out of 3 games my LoL client has crashed after I alt-tabbed either in loading screen or right at the start of the game. I alt- tab and then the client doesn't let me alt-tab back in without a lot of difficulty and when I finally do the screen is black and then I can hardly alt-tab out and I had a hard time getting task manager to come up to delete the current LoL client before restarting the client and coming back in. Also in mid game in my most recent game this happened and we lost because of it. ( I didn't ult tab that time either..)