In order to display polytonic Greek (Greek with accent marks and other diacritics), you will need a Unicode Greek font. You may already have a nice Unicode Greek font included as part of your Microsoft Windows font set (for example, Arial Unicode MS contains a very nice Greek font).

If you do not have a Unicode Greek font, you can download and install the free TITUS Cyberbit Basic font, which is available for both Windows and Macintosh:

TITUS Cyberbit Basic Unicode Greek font

Instructions are provided at the TITUS site for downloading and installing the font; if you have any questions or problems, let me know and I will do my best to help you with that.