Did you find this setting in the “Regional and Language Options” of your Control Panel? The language you select there is called the “input language” and it specifies a particular “code page” which maps your keystrokes and keystoke combinations to the characters displayed on the screen. English-US is code page 437. Greek is 737.
More than you’ll ever wanna know is here:
http://www.i18nguy.com/unicode/codepages.html