Press ctrl+alt+delete twice, enter Administrator in the name box, and if you didn't specify a password for that account when you set up Windows, voila!
Other than that and wiping your HD, we can't help you.
there are programs and stuff out there to get past it but dont think its legal.. you could boot into safe mode and in run type "control userpasswords2" this will allow you to change the passwords.. i no this works on vista unsure bout Xp
control userpasswords2 works for XP, BTW. I use it to make my computer automatically login, while still retaining a password for things like Lock, screensaver and standby.