How to read CPU temperature into Arduino
It is a lot easier to read directly from the system. This of course depends on what operating system the computer is running but this can be easily read using terminal commands (and most likely easy with cmd prompt). For example on OSX You can get temperatures using istats and on linux you can use lm-sensors

My suggestion would be to run a Python script that executes bash commands to read the cpu temperature and then send the data over serial to the arduino. 

I don't think using your own sensor would be a good solution seeing as it would be messy and would probably give less accurate results than the onboard sensor.