That is the hard drive activity LED. It "flickers" when there is data being read from or written to the hard drive. I don't know of a program that will emulate that light, but there are several Hard Drive monitors that show a graph and show activity. As far as your machine halted, I assume you mean hanging up? The little spinning wheel that takes place of the mouse pointer will let you know if it has or not. It's not terribly important that you keep a close eye on the HDD activity light honestly, but if you still want a hard drive monitor, I'm sure we can find you one.