Help - i can't log into my computer! need to reset password while it's locked...?

you know how if you're logged in, you can't lock your computer

well i have it locked but have forgotten the password :S

is there anyway i can get into it???
Update: ok i meant to say CAN lock your computer...

i've tried the F8 thing and it doesnt work...

i can't just give up and get a new computer cos it has all my pictures and EVERYTHING...
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