Sensible, I assume you do not really talk about your computer name. If I understand correctly what you say, when typing the first letter of your username on the posting page, the PC offers you the wrong (your mis-typed) name.
If this is the case, in your C
WINDOWS directory is a file with the ending PWL, you can search it with Explorer using *.pwl
Now, listen to this first: YOU CANNOT EDIT THIS FILE! To change the passwords, usernames etc. YOU NEED TO DELETE the file. Windows will on next start-up create a new one.
BUT!!!! YOU WILL HAVE LOST ALL PASSWORDS!!!
Now it is your turn to decide.