The dual boot option is actually a Windows file
Go to system properties, click on the advanced tab, at the lower portion of the window click on 'settings' in the Startup and Recovery section, at the top, click on the EDIT button, in order to have a single boot, you should have this
[boot loader
timeout=1
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOW
[operating systems
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /fastdetec
if you have another entry under Operating Systems, delete the line and then save the file
In this instance it also reduces the delay to boot to 10 seconds.