Bootcamp function key problem, suddenly

For 5 years, I've been happily using BootCamp with Windows 7 on my 2011 iMac 27" i7 with an extended Apple keyboard.

The subtitling program that I use - which is the reason I'm working in Windows - uses the function (F) keys for various central functions. A few days ago, the function keys all of a sudden stopped working as they used to. In the subtitling program, they now only work if I also press the fn key, whereas before they worked without the fn key.

Before, when I pressed the fn key and some of the function keys, I could access other function to control sound and display brightness, both in the program and on the desktop. That's no longer possible.

I have tried with another, similar, Apple keyboard. The result was the same. In OSX, the keyboard works as it should.

I've tried to download an update to BootCamp - following Apple's guidelines - but updating BootCamp is apparently no easy task judging from the many frustrated comments on various websites. I also didn't succeed, and anyway, of course, wouldn't know whether that would make a difference.