With Windows XP, you can change an account password without knowing the original password. I should mention that this information could be used to get yourself out of a jam, but also for bad by those shady enough to do it. In Windows XP, go to yoru command prompt and type the following command:

net users Administrator *

or for a username with spaces:

net users "Bob Smith" *

From there, it should ask you for a new password. You can enter it, but pay attention to what you type because the letters will not output to the screen. Confirm your password choice. When done, it will tell you that you were successful.