|Is there just one user on the machine?|
Is it saying invalid password?
Does anybody else have access to the machine? Because if you haven't changed the password, then most likely reason is somebody else has!
If its caused by a corruption to the windows system files (eg, the file that holds this information, SAM), then if you have a vista install DVD you could boot from it and use its repair mode. This will give you access to system restore - where you could try to restore to earlier date.