@aaronwee, i also encountered with OS crash for my macbook pro, last time when i installed google chrome into my OS, i using macbook pro 13 inch and i check that my harddisk have some sound coming from it.

i think it was harddisk problem and i have send it for repair( change into new harddisk at VR technology) and still dont get it yet. do your harddisk have small sound coming from it? if yes maybe it was your harddisk that make your OS crash,

and also do you upgrade your ram? for TS just like Ojimaru said, try to run disk utility and do check your harddisk sound( if there a small sound on it then maybe your harddisk got problem( eventhough it hard to hear harddisk sound from i mac i presume).