(specifically, HP's Pavilion P6-2310)
Excerpted from http://distrowatch.com/weekly.php?issue=20121126#news

Boot machine while pressing F10
Find Secure Boot in the menu tree, ignore warnings
Disable Secure Boot feature
Enable legacy boot options
Enable specific legacy devices, such as USB devices
Save and reboot while holding down F9

If this doesn't work for your device, refer to Jesse Smith's article for clues.
Also, post your solution if some other scheme works for you.