For Windows, "systeminfo" when a command prompt is open, is the most accurate time. The pagefile is being updated more than one would think and the System Idle Process will become more inaccurate the longer the amount of time the computer is turned on for.
Systeminfo can also be done over a network.
I know that 3rd party applications were a little out of the question. But, if I can dig it up, I'll post a link to a little program I made that when used just uses systeminfo either locally or over a network and then saves the results to a specified folder and also pulls the results to the screen using notepad.