To boot a compaq Deskpro sff without keyboard, you need to configure the BIOS in a special way.

I found this:

– Run BIOS setup by pressing F10
– Goto ‘Security’ and ‘Set Power On Password’
– Type in a password and F10 to accept the change
– As soon as password is set, ‘Password Options’ will appear under ‘Security’ tab
– Enable ‘Network Server’ mode in there
– F10 to save changes and exit

When booting up it won’t ask you for F1 anymore. However, if you or
someone else plugs in a keyboard it will ask for a power-on password.