Most users will be required to log on to Windows Vista with a password each time they start their PC, allowing many different users to each have a unique login username and storage area.
If you wish to change your username at some point, go to the control panel by clicking the Start Menu and then Control Panel:
Then click User Accounts and Family Safety:
Then select Change your Windows password:
Then click Change your password:
Now, enter your old password, followed by your new password (twice) and then click Change Password: