Altgr randomly stops working on windows 8
There are many blog posts out there, which try to explain what the issue could be. Many times specific programs are the cause of the problem.
For me it was a
Windows Remote Desktop connection I had running, that prevented the key from working. I didn't have to close it, just opening the window and clicking anywhere inside it somehow made my [AltGR] key work again!
EDIT: It seems to the related to the full-screen mode of RDP. See this KB article.
Just try to close all your applications one by one to find out what could be the cause.
Here it turned out to be a running VirtualBox-instance that had tampered with my keyboard setup. After shifting focus to the VM and back to the editor the AltGr-key worked as expected.
I'm running Windows 7 SP1, so AltGr acting as Alt is a problem not only on Windows 8.
For me, it was caused by Hyper-V (Microsoft virtualization). When I go to virtual machine, press AltGr it works. Return to desktop, works again!