Bumping this thread because I've been having this same issue happen to me for a while now.
I am using a nvidia GeForce 540M video card with the latest drivers (which I have updated each time new drivers are up and the problem has persisted with each update), Windows 7 64-bit, and a 2nd gen Intel i5.
The hard freezing with the horrible sound repeating (the last split second, sounds like a buzzing) that requires me to hard shut-off my computer is whats happening to me. It occurs completely at random... sometimes right in the first couple minutes of my first game, or sometimes after I've been playing for hours.
Sometimes if I mash alt+control+delete when it first happens I will get lucky and bring up the task manager, where I can just close LoL and restart it... and sometimes if I wait around for 10-15 seconds it clears up, but that's only happened a couple times relative to the dozens of times my comp has froze up playing LoL.
I've played other much more demanding games as well on this computer with no issues. Only LoL causes my computer to freeze like this. I'm going to try running the game in XP service pack to compatibility mode and see if that helps.
Any suggestions from a red would be much appreciated!