Windows: When it comes to diagnosing hardware problems, there's not such thing as too much information. HWiNFO gets credit for trying to push that limit, though.
The app analyses your system and provides loads of information about your hardware. Where it differs from apps like Speccy is that it also allows you to see a live view of system sensors like SMART, as well as keep an eye on your system temperature and voltages. For most users, HWiNFO is probably overkill, but if you're heavily into hardware tweaking, it can be a great aid.