Please can anyone tell me how to bypass this windows logon screen for window vista and XP??

I forgot my password on Windows user password and was wondering if their is way I can pass this mexx without formating!

Though I heard for window XP all you need do is hold down the shift key soon as you on your system and you will bypass all start up procedures, (haven't tried it though)

So, please I need know how to bypass windows logon screen for Vista.


Also, for windows vista and XP is there a way I can truely secure my computer from that (holding the shift key down stuff)
I need protect myself from some bloody minded hacker

THANKS C YA