* Pre-UEFI, before 2008 for most models.
With the computer turned Off, press the power button to turn On the computer, and repeatedly press the F10 key. Normally, the Compaq or HP splash screen will display a message to press F10 to enter the BIOS just before the BIOS menu is displayed.
* UEFI Enabled, 2008 to mid 2011 for most models.
If the computer has the UEFI pre-BIOS environment, when the Compaq or HP splash screen displays, you are prompted to press Escape to enter Start Up Menu. Then there is a prompt to press F10 to enter the BIOS Setup.
Because of the different types of CPUs and BIOS configurations, it may be possible to turn On the power and press F10 for the BIOS without going through the UEFI Start Up Menu.
* Shortened Startup without splash screen, mid-2011 or later for most models.
To help computers start faster, on some models the Compaq or HP splash screen is not displayed during the start up process. In this case, after pressing the power button, repeatedly press the Esc key to enter the Start Up Menu, and follow the prompt to press F10 to enter the BIOS Setup.
Source: HP website