I do not think it's always F8. Sometimes it's F2 or F12.
My motherboard boot screen shows a little guide on what buttons do what. F6 is my safe mode thing.
Kon Boot
if you had linux you could go into recovery mode, get a root prompt, and do: passwd [username] and then enter your password.
If you have Gateway and don't minding restoring your computer, boot it up and immediately press the Alt key and repetitivly strike the f8 key for about 5 seconds. After that it's self explanitory.