It would sure help us to know which operating system the computer is using? XP HOME, XP PRO, something else?
Have you scanned for spyware? If not, please do.
In addition to Ray's good suggestion, it's possible you're typing in the wrong password or the profile has become corrupted.. Try restarting the computer into Safe Mode, the type the user name as "administrator".. If you've not created a special password for the "administrator" log in, then leave the password blank and press the "Enter" key.. That should load the administrator profile from which you can then change the password on your personal profile..
How To Start In 'Safe Mode'
If you're using XP, after logging in as "administrator" in Safe Mode, click on Start-Run, type: "CONTROL userpasswords2", then click on OK. When the "User Accounts" window loads, click once on your username, then click on "Reset Password" and follow the prompts to reset your password. Reboot and try the new password.
If that doesn't fix the issue, then you probably need to create a new profile which can also be done from the "administrator" login.
Hope this helps.
Grif