I can help you with WinXP if you have it.
In the taskbar in the bottom-right corner you should have a small icon with an abbreviation of the writing system you are using (mine says EN for English). Right-click on it and left-click on 'Settings...'. A small window will appear called 'Text Services and Input Languages'. On the right side left-click on the 'Add...' button. An even smaller window will appear called 'Add Input Language'. Under 'Input language:' select 'Greek' (or any other language, it doesn't matter, the next step matters). Then check the box 'Keyboard layout/IME' and select 'Greek Polytonic'. Then click 'OK' and again 'OK' to close the windows. Now, on the bottom-right corner if you left-click on the EN icon you should have a list of all the 'Input languages', on of them being Greek. When you open a writing program like Word or Notepad to write in Greek either left-click on the icon and select GR, but also Left-Shift + Left-Alt, or Left-Shift + Left-Ctrl (depends on the settings).