Startup

Have a look at easyBCD to eliminate second boot option.

Right-click My Computer, and then click Properties.

On the Advanced tab, under Startup and Recovery, click Settings.

A Startup and Recovery dialog box will appear.

Under System startup, click Edit to modify the Boot.ini file.

I recommend you make a back up of this before editing it.

Delete the OS you want to remove, it will look something like this:

"multi(0)disk(0)rdisk(0)partition(1)\WINDOWS.0="Microsoft Windows XP Home"
/fastdetect"

Save and exit.