Your computer thinks that you're using an 'International' (or possibly 'US') keyboard, rather than a 'UK English' one. The way to fix the problem varies slightly across different versions of Windows but hopefully the following (for Windows 7) will be close enough to the specific solution that you need for your own version of Windows:
Click Start.
Click Control Panel
Click Region & Language
Click the Keyboards & Languages tab
Click Change Keyboards
Select 'English (United Kingdom)'
Click Apply
Click OK