Someone went into your computers bios screen and screwed with the password from there. What a lot of people dont know is there are two passwords for windows XP. the one that everyone is used to like at the login screen after it starts up and all that, and then one before all of that, which is the one troubled is describing. The common password is an easy fix. I'd say start XP in safe mode and log on as admin and change the password from there. But I dont know what do do in this case, sorry.
I've used windows XP before and I have to say, it's pretty complicated.
You could try different ways to type in a password. Like the person suggested above, try putting your computer in safe mode and log in as an adminastrator. You can change your password that way. I'm not even sure what else to tell you either.
It's kind of hard to say.