Delete the old OS boot option from the OS boot screen

For windows 7 and higher versions:

1.open the command promt with administrator privilege.
(Goto C:\windows\system32 and right click on cmd.exe, run as administrator)   
2.Type bcdedit command 
 (you will get a list of the OS boot files)
3.copy the IDENTIFIER of the OS you want to delete from the boot menu
4.Type bcdedit /delete IDENTIFIER 



For windows XP and lower versions:

http://www.howtogeek.com/howto/windows/how-to-delete-modify-or-disable-an-incorrect-or-duplicate-entry-on-the-xp-boot-menu/