Why won't my computer let me run scheduled tasks?

My computer is in desperate need of a defrag, disk cleanup and other maintenance. But everytime I try to perform these tasks, it asks for a password. I have even tried changing the password, but it keeps saying "access denied". It has never been run and under the status it says "could not start". WHY? I am running Windows XP if that helps. I have even tried going through the Scheduled Task Wizard and when I get to the last screen that asks for a username and password, once again I am at a "STUCK" point because it keeps saying "The new task has been created but may not run because the account information could not be set." Access denied! WTF? I didn't ever set the account information to begin with so how can I be denied?