About the greek font. In order to be able to type using the greek alphabet, go to your Control Panel, double-click Regional Options and Language Settings (or something like that), go to the "Languages" tab, click on "Details" for XP or "change keyboard layout" for Vista I think, click on "Add Input Language" and just select Greek. That should do it.
If you can't find it, maybe this website will helphttp://windows.about.com/od/windowsvistatutorial1/ss/keyboard-layout_4.htm
Shows you how to do it in 6 steps.
Now, the phrase you posted means "Won't you release the friend?". "Don't release the friend" would be "Μή λύσεις, ὦ ξένε, τόν ἐν τῆ οἰκία φίλον". "Won't you not release the friend?" would be "οὐ μή λύσῃς, ὦ ξένε, τόν ἐν τῆ οἰκία φίλον;".
When you use "οὐ" it has to be in reference to a fact or an opinion, not to a wish or an order. For example an ancient would say "It's not easy" = "οὐκ ἐστί ῥάδιον", but never "μή ἐστί ῥάδιον", which would mean something different and would be used in different settings. Therefore, it's easy for instance to understand that you can use only "μή" and not "οὐ" to indicate negation when using the imperative(=order).
Which can be a really useful hint... For example if they ask you whether "λύετε" in "λύετε τούς φίλους" is indicative or imperative there's no way to know... But if they ask you the same question for this sentence: "μή λύετε τούς φίλους" you'll know it's imperative and this sentence is an order- "don't free the friends". "οὐ λύετε τούς φίλους" would be an acknowledgment- "you're not freeing the friends". In more complex structures like those used to indicate hypotheses or in the indirect speech things of course get messier but that's another chapter.