I think you may have accidentally activated the Filter keys. Go to Control Panel and Accessibility options.
On the Keyboard tab, see if there is a tick in the Filter keys checkbox. Click there again to remove it. Click on Apply and check to see if the problem has gone.
I am sure there is a short cut to activate this because I have accidentally activated it myself in the past, but I don't know how.
If this doesn't cure it, post again.