Windows 10/8/7 has an inbuilt Memory Diagnostic Tool. Most of the time Memory Diagnostic won’t detect a bad RAM. To be honest, if you want to check the stability of RAM, Windows Memory Diagnostic is not recommended. You could instead, use an open-source memory test software called Memtest86+.
Advanced Memory Diagnostic
Memtest86+ is designed to test and stress test an x86 architecture computer’s random access memory (RAM) for errors. You can run the Diagnostic in three different methods. Using the Floppy drive, CD ROM, and USB. This tool comes helpful when can’t boot the OS, and you think it might be a bad RAM.
You could download the ISO image or any other format from their website and burn to disk. Then boot the Computer using that disk to start the Diagnostic.
Just remember MemTest86+ doesn’t run on Windows, it’s a tiny Linux as it is with a GUI.
It’s recommended to run the tool over-night i.e. the Memory test should run at least 7 to 8 passes to get the actual results. Even we get a single error means we have defective RAM.
You might want to check out some more PC Stress Test free software for Windows too.