Thanks for the help! I have dug out backups from a few weeks ago and restored from them, and everythiing is (so far) behaving. I checked to see what the HID service was set to and it is still automatic, so quite what has altered or changed I have no idea, but at least things are now working...
I was wondering if anyone could help me. A few days ago, I found the media keys on my keyboard stopped working. I tried the keyboard on my laptop, and everything was ok, so reasone dit must be my desktop. I traced the problem to the HID service- on my laptop it was runnig, on my desktop it...