help booting to a compaq

[Lenny]

Hi,
I am trying to get this Compaq to boot to the USB drive.
It is a Compaq f750 with Vista running on it.
I should be able to hole down one of the F-keys on start-up, but trying 
this, and a number of arrows down on some of the F-keys has yielded no 
successful boots.
Does anyone here know how Compaqs typically offer the temporary boot 
options?
Thanks.

Glenn