Thanks for the suggestion, Steve.
Power button's behaving as it should again. Here's what I did:
- Create a control panel shortcut that allows it to be run as administrator
(from: http://www.intowindows.com/how-to-run-control-panel-as-administrator-in-windows/)
- Open Troubleshooting\Power and follow the on-screen instruction
Hope this helps other users with the same issue.