To remove a start up os option, open your system properties, go to advanced, in 'start up and recovery' click settings, then edit and remove the line for the os you have removed. The first line under [operating systems] will be the first one you installed and so on. You can also change how long it waits at that screen. Great way to hide an o/s it set to 0 seconds for daily use and change it when you want to load in to the second one. Most people would never know it was there. When modifying the os choice text file be very carful not to touch the first option as your system may no load anymore.