I have just recently bought myself a HP ENVY DV6 laptop, and a wireless Xbox360 controller. The controller is made by Microsoft so it isn't a cheap 3rd party ripoff. I went to play Oblivion and about 1/2 of the buttons weren't working, including the z-axis. I went to the game controller settings and the buttons all seemed to work fine, but the z-axis was still not working. The laptop is installed with windows 8 (it came with the laptop so there wasn't ever win 7 to start with), and I've heard that heaps of other people are experiencing the same problems with Win 8.
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If you heard that heaps of other people are experiencing the same problem, then I guess there's not much you can do except wait for updates to the controller drivers, for Win 8.
So Windows 8 has botched the proprietary Xbox controller drivers? Write an email letting them know and they should patch it relatively quickly.