Here’s the way I use to change the user name and image in the Vista start menu.
Click the Windows button (start button) in the task bar and then click the user picture.
This next screen opens, click the option you want to change.
If you chose to change the picture you’ll get a window that is very easy to figure out.
If you chose the change the account name, Next you’ll see this dialog, click on continue.
Here’s where you change the name.
Changing the user name will not change the disk drive folder name, DO NOT go and change the folder manually you’ll break your Vista installation in many ways. For more information and links to instructions on changing the user folder name see this post, How to Change a User Account Name in Vista at the Vista Forums site