How do you remove a boot screen password?

when you start your computer, it matters what compnay your PC is from, weather it is a DELL or Compaq or HP, when you start it up the PC company comes up before the Windows Vista or XP screen, right away at the company name it will say BOOT=F12 or what ever the company puts as its boot button, right away press the button it says, the BOOT MENU will come just scroll through the options and find the system passWord, change it, if this doesn't work you'll have to reboot your PC.