So you login to your computer every single day, but there's more
than one account to choose from… either because you got the
computer from somebody else, or some software package
added a user account that you really don't want to see.
So how do we hide that other account from the login screen?
There's a simple registry hack that you can do to hide accounts
from the login screen, or you could alternatively delete the
user account if you really don't need it. Both methods are
listed below.
Manual Registry Hack
Open up regedit.exe through the start menu Run box, and
then navigate down to the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\
CurrentVersion\Winlogon\SpecialAccounts\UserList
On the right-hand side you'll see a list of keys that correspond
to user accounts that are considered "Special" and are not to
be shown on the login screen.
Create a new DWORD value with the same name as the user
name that you want to hide. You'll note that the actual
username in the illustration was "OtherPerson" and not
the description that was shown on the login screen.
To unhide the account, simply delete the registry key that
you created.
The next time you log off you'll see that the other account
is gone.
Hide User Account with Tweak UI
You can also use the Microsoft Tweak UI PowerToy
to do the same thing. (Thanks to Gustav for reminding
me to include this)

Just click on the Logon key in the left hand side, and then
uncheck the box for "Show <username> on Welcome screen"
Login to Hidden Account
You can still login to the account even though it's not shown…
You'll just have to use a little trick.
At the login screen, just hit the Ctrl+Alt+Delete
combination twice in a row, and you'll be presented
with the old-school login screen, where you have
to manually enter the username.
Type the name and password of the other user, and login.
Delete User Account
Alternatively you can always delete the user account if
you are absolutely sure that you don't need that user
account anymore. Right-click on "My Computer" and choose "Manage"
Navigate down through "Local Users and Groups" and
then Users, and then locate the username in the list
that you'd like to get rid of.
Just right-click and choose Delete. Note that this
step cannot be reversed, so make sure you only
delete accounts you really really don't want anymore.
If you are unsure, you can just hide them using
the first option above.