Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Many fonts you already have installed will have Greek letters within the font, but since they aren't represented on the keyboard, you need to copy them from your character map..Δ Σ Ω ... (see, it's easy!).So...click Start/ All Programs/ Accessories/ System Tools/ Character Map and it will open up a menu with all kinds of symbols, including Greek letters. Select the ones you want, copy them and paste into your graphics program.
If you use Illustrator, open up Glyphs pane, it is under Window, Type, Glyphs. You should find most of them there and the rest of the special characters for a specific font. Some fonts do not have Greek letters included.