By "logon" do you mean:
a. logging on locally at the Vista computer?
or
b. connecting over the network to another Vista computer?
If you did not define a password to the user account created during initial
installation and have not created any new user accounts, then logging on
locally should be automatic, so I'll assume you mean b - connecting over the
network to another computer. If your user account does have a password or
you have defined additional user accounts, the user name(s) will appear on
the Welcome panel.
1. to find out what user account you are currently logged on with:
a. Start, Run, key
cmd
press Enter
b. key
set username
press Enter
2. by default, you can not connect to a Vista computer over the network
using a user account that does not have a password. This is the same as for
Windows XP (at least XP Professional - not certain about XP Home). For more
information about this see the thread "Partition sharing through network" in
the newsgroup microsoft.public.windows.vista.general.
3. to create a password for the currently logged on user account, press
Ctrl+Alt+Del, select Change Password. If this account does not yet have a
password, just leave the Old password box empty.
4. to view, add, delete or change user accounts (e.g. set the password),
either:
a. Start, right click Computer, select Manage [click Continue OR key an
adminstrative user account and password], expand Local Users and Groups,
click User
or
b. Start, Control Panel, User Accounts, Manage User Accounts
or
c. Start, Control Panel, User Accounts, Give other users access to this
computer
Note: usernames are NOT case sensitive, but passwords are.