Basically, from an Elevated Command Prompt, (Start, Run, {type} cmd {press CTRL + SHIFT + ENTER)
Run bcdedit to show a list of bootable OS's:
C:\Windows\system32>bcdedit
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=C:
description Windows Boot Manager
locale en-US
inherit {globalsettings}
default {current}
resumeobject {a6212382-f69e-11de-ab6a-b9a2e78df1ca}
displayorder {current}
toolsdisplayorder {memdiag}
timeout 30
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path \Windows\system32\winload.exe
description Windows 7
locale en-US
inherit {bootloadersettings}
recoverysequence {a6212384-f69e-11de-ab6a-b9a2e78df1ca}
recoveryenabled Yes
osdevice partition=C:
systemroot \Windows
resumeobject {a6212382-f69e-11de-ab6a-b9a2e78df1ca}
nx OptIn
C:\Windows\system32>
and then type:
Bcdedit /delete {boot loader identifier}