Something strange has just happened to my keyboard

I was typing a post on another forum, when I left the keyboard for a few minutes to see how much snow had fallen in the last hour, and when I cam back, I tried to type a pound symbol(£), so I pressed shift+3, and got a hash instead of £.
All letter keys work perfectly, as far as I can see, it is only the 2 and 3 keys which dont work.
2 gives me the @ symbol, whilst pressing the @ key gives me ".
3 gives me #, whilst pressing that key gives me \, shift pressing the # key gives me |.
What has just happened?
The keyboard is part of a keyboard/mouse set, the Microsoft Wireless Optical Desktop 700 v2, and I am running Win 7 Home Premium x64.

I have solved it, and that wasn't the cause, I checked about five times.
The cause was that I had, at some point during the day, pressed ALT+Left Shift, which switches the keyboard layout, despitre the only layout shown in the second screenshot you posted being United Kingdom.
Pressing ALT+Left Shift again solved the issue.