Yeah you don't need to know their password to reset or delete it, as long as you are an admin.
However if you completely locked yourself out of windows, theirs still a way.
First off you can use a password reset utility. Emergency Boot CD seems to give me a fairly high success rate. You can download it here http://ebcd.pcministry.com/
and ask for help if needed.
Second off, you can try a password cracker
Someone like me can get 95% of windows passwords in 5 min (And 99.9% within 24 hours
). My favorite for this is ophcrack, as you can download it, boot off the CD and it will do everything for you (however, has extreme problems with complex passwords)