Is the Administrator account password-protected as well? Check by hitting ctrl+alt+del at the login screen...
If not, Geek Squad can do it, it takes them around 5 minutes.
It sounds like you need to reinstall XP from scratch. Back up any important files and use your recovery discs (if you have them).
In my experience, installing a service pack on an unstable OS causes more problems than it solves. A...
All computer manufacturers provide a 1-year warranty. Did you read the sheet of paper or warranty booklet that came with your laptop? It tells you the full terms of your warranty, and often provides instructions on how to extend that...