In the GRUB boot loader, you get asked to select an operating system in an allotted amount of time and, if no selection is made, a default operating system is selected for you. Turns out the wait time can be altered and it can be altered to be indefinite so that it never selects a default, GRUB will always wait for a selection before booting. To adjust this, edit the /boot/grub/menu.lst file as a root user (or use sudo). In the file, look for this section:

## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined)
timeout      30

You can edit the timeout value (30 in this case, your number may vary) to be any value you like. If you'd like to make it so that a default selection is never made by GRUB, you can enter a very big number, or even better, you can just comment out the entire line putting a # in front like in the line above it.