Windows on an iMac: unable to change keyboard layouts
I had this problem and fixed it by: Control Panel -> Regional Settings -> Keyboard and Languages (tab) -> Change keyboards (button) -> Selelct the top language added: (English United States) -> then click "Add" -> Navigate between the options to (English United States) -> click on "keyboard" -> now check the option "US"

This solved my problem, I hope it solves yours