Solution
Reboot the machine. When the company name, Compaq, displays in big red letters on the Presario's screen, a cursor should appear in the upper right hand corner of the screen. Press F10, and instead of booting normally, you should see the setup screen for the Compaq's BIOS.
Someone, in response to seeing this page, told me that some Compaq machines have F12 as a BIOS setup key. If F10 brings you no joy, it's worth a try.