For all versions of Windows, just look for the "Start" button which is generally found on the left, bottom corner of your screen, depress "Start" and go  Start > Settings > Control Panel, then look on the gray bar along the top of the Control Panel folder and find "Help" which you should depress and see "About [your version]" as in this picture of a  Windows NT:

Here are some other ways to figure out which version of Windows you have:

If you’re using Windows2000, you’ll see that written in the box that you type your logon name and password in every time you restart your computer.



For WindowsXP, WindowsNT and Windows98, just depress the Start button and tilt your head to the left. You’ll see this info written sideways!



Another method is to find the “My Computer” icon on your desktop.
Right click the mouse on it and then click on “properties”.  The information will be given there.

I’m sorry I don’t have any information on typing Persian on other versions of Windows which I don’t have access to. I know the FarsiLang patch works on WindowsME and Windows95,however I’ve not had the chance to try out either.