In the Windows installer don't forget to select to install the spell checker for "Armenian".
Install the package armtex using MiKTeX's Package Manager.
If LyX doesn't correctly accept your keyboard input, open the menu Tools → Preferences try to use a keyboard map as shown in this screenshot. Then click Save and restart LyX.
Testing the Installation
Run LyX and press Ctrl+N to open a new document. Write a few words in English, and press Ctrl+R. A PDF viewer program should be opened; check that it displays the text correctly.
Open another document and call the menu Document → Settings. Select there in the Document class drop-list the class article (Armenian), select there in the Languages section Armenian, press OK and type in a few words. Press Ctrl+R and check the output.
If you do not see Armenian, check in the LyX preferences if the selected screen fonts support Armenian.
In LyX's examples folder you find the file armenian-article.lyx where you can see how Armenian can be used together with Latin characters and other languages.
It is possible to add new templates to LyX. Place the template files in ~:\Documents and Settings\<username>\Application Data\LyX2.0\templates. Then reconfigure LyX by using the menu Tools → Reconfigure and restart LyX.