Może to ci pomoże.
Fix #1: Open msconfig
Using the msconfig utility tool allows you to select which version of Windows you’d like to boot directly into when you restart your computer.
Follow these steps:
- Click Start
- Type msconfig in the search box or open Run
- Go to Boot
- Select which Windows version you’d like to boot into directly
- Press Set as Default
- You can delete the earlier version by selecting it and then clicking Delete
- Click Apply
- Click OK
- Restart your computer
If you choose to delete a version from the Boot tab of msconfig, that Windows version won’t be deleted, but it won’t be displayed on the Windows Boot Manager screen.
źródło: https://neosmart.net/wiki/remove-dual-boot-menu/
Tylko to nie usunie całkiem poprzedniej instalacji Windowsa a tylko wpis z bootloadera.