I could make that program if i had arma
but i dont, you can try this though, im assuming its in the registry somewhere so do this
go to start >run> regedit
then search (crtl+f) and search for armed or something along the lines of that. look through untill you find something that resembles a serial #, after that right click it and modify it. be sure if the serial number is surrounded by " or something, dont delete those, just replace the actual characters.
i hope this helps.