METHOD 1:
First enable "Show hidden files/folders" and disable "Hide protected system files" options in Tools -> Folder Options.
Now goto C:\ in my computer and remove the READ-ONLY attribute of "BOOT.INI" file.
Now edit this file and set the timeout to 0.
METHOD 2:
Right-click on MY COMPUTER icon and select Properties.
Goto Advanced tab and in Startup & Recovery section, click on Settings button. It'll open a new window, in which click on EDIT button, it'll open the same BOOT.INI file.
Do same as I told in METHOD 1, set the Timeout to 0.
Hope It'll help...