For Windows 7:
Click the Start menu button and then go to Control Panel.
Go to Clock, Language, and Region.
Under Region and Language options click on Change keyboards or other input methods.
Click on the Keyboards and Languages tab.
Click on Change keyboards... button.
Click the Add... button.
Scroll down and select the appropriate Greek keyboard.
Click the Apply button.
This is what you should see on your screen:
For more details, watch this video : Windows 7 Greek Typing
This is the Greek keyboard layout on a standard English keyboard device.
For intonation marks go to this article:
How do you type intonation and other Greek marks with the keyboard?
Setting Greek keyboard settings on a Windows 8.1 computer