Have a look at your /boot/grub/menu.lst file. There is a line near the top specifying the time it will wait before taking the default option. It will look similar to this -
/boot/grub cat menu.lst
Usually you just use the keyboard, and select the boot option you want. It should take off and boot.
You can edit the timeout value, and set it short if you want. I have one machine set for 3 seconds. This system is set for 10.