I would use Autoithttp://www.autoitscript.com/forum/
as it can "read" the text values from most windows programs such as SpeedFan for temps, cpu load and voltages.
Autoit can also monitor the running processes or if a particular program (window/taskbar icon) is running.
There is a comms library for autoit that can send this data to the arduino for processing.
Detecting a computer crash - write a loop in your autoit program/script to send a serial byte in addition to the other values, when this stops being sent you know something is wrong (I would have a sounder here instead of an LED.
This all do-able, but take it step by step.
Get used to using autoit and reading values from a program (Speedfan or other) window first.
Then write an Autoit program/scipt sending data using the Autoit Comms lib to test sending serial data to the arduino, something simple like a control byte to turn an LED on/OFF.
Once you have those working, you can work on the complete program sending monitored data to be processed by the arduino.
Search the Autoit forum for Arduino projects by others and if you get stuck ask, someone will help you out!.