Edit: SOLVED: It was the Xbox Gaming DVR, read below posts. ----------------- I have two problems with the latest win 10 update (th2), the Home/Guide Button on the Xbox 360 and Xbox one controllers does not work anymore with the latest drivers that installed this update. Now i can't open Steam, also i cannot use it on the the gamepad to keyboard emulators like JoyToKey, pinnacle, etc.. Also my Start menu resets the pinned tiles size sometimes after a restart. I can live with the start menu resetting, but anybody have a copy of the previous Xbox One usb controller Win 10 drivers, please . I have installed the old 360 drivers and the guide button works again, but i don't have the old xbox one usb drivers . Online there is only uploads of the very first win 7/8 driver, but that's too old. I confirmed some people also have noticed the same on the steam forums.
If I remember correctly: Open the Xbox apps and go to the settings menu, then enable the video recording feature. After enabling it, disable it again. That worked for me before.
Edit: Never mind, turns out that the button is really eaten by the Gaming DVR like you said, but now is per app, i already had enabled/disabled the DVR on the Xbox app and didn't worked, but now i re-enabled the DVR on the Xbox app and disabled the "use Guide button to open the DVR bar" on the DVR overlay that comes up when you press Win+G or the Guide button on JoyToKey/Steam and now the button is "free" again, thank you. For my second problem, i unpinned all my start menu apps and re-pinned them again and seems to work fine. I also updated again my 360 drivers, so my Xbone drivers and 360 drivers are up to date and the Guide button is working.