How can i log onto my vista PC as i don't know the pword? any methods ways without leaving footprints?

you see, i'm active and do a lot of stuff outside and play 3 sports but I'm only allowed an insanely low 15mins on my computer and my parents put a pword on to for no reason, sad... really. so, how can i find out the pword and access my PC without deleting the pword or having obvious traces? thanks