I purchased a Logitech touchpad wireless K400 keyboard to replace the Lenovo remote keyboard.
Even though I had Sleep set to Never in Power options when I hit the power button on the K400 it put my HTPC to sleep. I'm running Win 8.1.
It would not come out of sleep mode or boot back up to the OS. I've read that this is a common problem with Win 8. After pulling the plug and letting it sit for a few minutes it booted up and reset the bios. It would then boot up to the OS but would not completely power down the computer. Read that turning off Fast Start in Power options might fix it, but it didn't. Only reflashing the bios solved the problem.
One other problem I had was with Logitech's Setpoint software, the same one the OP in this Logitech forum thread had, no mouse or keyboard function tabs. The software is optional, my keyboard still works, but the software gives you more control.
http://forums.logitech.com/t5/Keyboards-and-Keyboard-Mice/No-functons-in-Setpoint-K400/td-p/903250
The solution is on page 2 of the thread in post 4. Turn the K400 off, using another mouse/keyboard open the Setpoint software, under the Tools tab open Unifying Software, Advanced tab, highlight Keyboard K400, Un-pair, then re-pair the K400. This should give you the mouse and keyboard tabs.
Now the power button on the K400 powers my HTPC completely down.