If Vista is in control, you need to tap the key sooner.

Different computers use different keys to select the boot order, or to get into
BIOS setup. In my experience, Del is most common, but other people may have
different experiences.

For what it is worth, both of my computers (HP desktop and ASUS netbook) use
the ESC key. It should tell you when the POST appears, usually along the
bottom. Some computers do not have this function at all.