Under the C:\ Drive is a file called boot.ini (it might be hidden I forget). This is what displays the list when you select the OS. You can simply delete what you do not want to show. Then once you reboot you won't have NT anymore.

Alternatively, you can right click on My Computer, and go to properties. Then click on the Advanced tab. Then click on Startup and Recovery. The first checkbox you see says: "Display list of OS's fo x amount of seconds." Well you can just uncheck this box so it will not display any operating systems.

Let me know if this helps