When I tried installing Ubuntu ISO image to my USB drive, the universal USB installer displays this message:
create syslinux MBR on (H:) - existing MBR will be overwritten
If I click next, will my Windows OS be broken?
No. It will simply make your USB drive bootable. It does not touch your hard disk(beyond downloading an reading the image downloaded). It will not mess up Windows boot. However, if your USB drive is bootable, the boot will then be Ubuntu. When you install fully, however, the bootloader will be replaced with GRUB , which offers a choice between Ubuntu and Windows, as well as many other OSes if they are installed.