a guest account won't do much good unless it has administrative access to the machine. do you have more than one user set up on that machine? (kids, hubby, etc?)
if you do, log in as that user.
Option 1: Go in to control panel (start, settings, control panel) double click on user accounts, find your user and click on it. click on create password/change password.
Option 2: right click on my computer select manage. click the + sign next to local users and groups, click on users, on the right side of the screen click on your user then right click and click set password.
if the user account you are logged in as doesn't have administrative access to the machine you'll get an error when you try to change the password. if it allows you to change the password, once you change the password log out as that user then try logging in as you with the new password.
if all that doesn't work, call your IT guy/gal at work and ask if he/she will come out and fix it for you. Be prepared to pay at least $50 unless he/she likes you.