I have installed (not live CD) Ubuntu on a USB stick, and now having dual boot with Windows. How can I use this USB on another machine, as the latter does not have grub.
How to make the USB standalone bootable without grub, working on any machine?
You need to have grub installed on the USB stick or the hard disk of the host machine. In your case as you want it bootable on any machine, you need it installed on the USB stick and boot the USB using BIOS option.
OBS: there is no problem on your machine to have GRUB on your HD (to dual boot with windows) and on the stick. If you boot from your HD, you'll have your usual options and GRUB on the stick will not be used. when you boot from the USB, GRUB on the stick will be used and you will have the options you defined there (probably only booting the Ubuntu on the stick as if you use it on many machines, you don't know what will be on these machines)
Installing GRUB2 on the USB stick:
Open a terminal and type: