I developed a cool application ,which I want to send out to users but may have to change the code later for all current owners.
Question is, how do i send out later, to owners with say, a Nano embedded in the device, a means to reflash with updated code?
Has anybody implemented a simple flash routine we could email to users, they could just load the windows app, connect to usb, and hit go? It really needs to be that easy if possible.
It is possible to invoke the AVR uploader from a command prompt, so it could be done from a batch file. You would need to provide the uploader and the executable image, and provide some means to launch the loader. You could wrap that in an executable file, for example an executable ZIP file, or supply it on a memory stick with autorun configured and persuade them to trust you enough to run it - or just give them instructions to run the batch file manually.
Well I will have to take your word on that, and do more research based on what you said, cause frankly most went over my head. 
Have little avr experience...
Ok, I found avrdude's page...
http://www.nongnu.org/avrdude/
So I would have to make a batch file customized to load the program via avrdude, to the USB port-connected Arduino?
Or would it have to be ICSP connected?
And would I supply only a hex file to be uploaded?