All check-boxes are unchecked in "Make the keyboard easier to use".Check your 'Accessibility' settings in Windows. There is a feature called 'sticky keys' which I've seen get randomly enabled in the past.
On my current Windows 7 PC, it's under Control Panel --> Ease Of Access Center --> Make the keyboard easier to use
It was called something different in XP, and may well be in 8 or 10!
Make sure Sticky Keys is disabled.
Or, if that's not the problem, try cleaning your keyboard