If you install windows after installing ubuntu, windows will replace Grub bootload with its own bootloader, which will remove the boot menu when you turn on your PC, removing you the ability to boot ubuntu.
Although you can always restore grub, i'd always recomend installing ubuntu AFTER windows. Whay i'd recomend now, is to reinstall windows in the whole drive, then re-install ubuntu and select "install ubuntu alongside windows" when installing ubuntu and the ubuntu installer will let you choose how much space from windows you want to take for ubuntu and it will do all the dirty stuff needed to get the dual boot working correctly.