I have Windows XP, and it keeps changing my password. Why?

It is not actually changing your password. It is just showing more "dots" than your password has characters. In older versions of Windows, if you had a 5 letter password, it would show 5 starts or dots. Since this allows someone to get closer to guessing your password because they know how long it is, they added a new feature to XP so that it just shows a bunch of dots every time. That way, it doesn't give a hint as to how long your password really is.