I am going to break my own advice here because you are using the wrong tool for
the job. Startup manager does not do what you want. There are several ways to go
about this. You could install a utility to edit grub through the GUI. There are
several, but the easiest way is to use a text editor and to edit it.

open a terminal and type sudo nautilus and press Enter. Type your password.
Navigate to /boot/grub and right click on menu.lst. Choose Open with Text
Editor. Open it.

Timeout is the number of sections that you want.

You dis not ask for this but you can change it to boot any listed operating
system by default.
Changing the number after Default changes which OS boots automatically. It is
zero which is the first listed OS. If you change it to the number for Windows
XP, then it will be your default OS. Or you could not change this number but
instead cut and paste the lines for Windows XP and move it ahead of the Linux
distros, thus making it zero. Grub uses ordinals so zero is the first and 1 is
the second, etc.