hacking my own computer?

a certain family member who got mad at me decided to cap my computer access (note this is actually MY computer, i'm not trying to get onto the family PC or anything.) totally uncalled for.

so the problem is not the login information, but there's a password input dialogue during the startup screen. is there a way to change/remove it? the computer won't do anything unless i input the password (i only got on because he put it in for me, but this can't go on...)

there are two accounts on the computer, mine and one "guest" account of sorts, they both have administrative priority. i have the login info for both. is there anything i can do?