VNC passwords on Windows systems are a popular topic of confusion amongst 
VNC users. At times, you may try to change your password, but will find 
that your "old" one still works!

The most common pitfall is understanding the difference between the "Default" 
and "User" settings. The default settings should possibly be referred to as 
global settings. These are the settings inherited by VNC whenever it is 
started whether as a service or an application. 

However, VNC also allows for the assigment of unique VNC passwords for each
Windows user on a machine. These passwords are set by logging in as a windows
user, and setting the password in the user settings dialog box. 

A great explanation with a diagram is available at: 

http://www.floydsoft.com/vncauth.html 

Thanks to Floyd Russell for the contribution!

Another perspective from RP:
----------------------------

"I changed the password when I was logged in on the server machine, which 
is running W2K Professional.  Then I logged out. Now when I connect to it 
when no one is logged in, the old password works and the new one doesn't."

WinVNC keeps a separate set of settings for each user who is logged in, and
another set for when nobody is logged in.  (This is so that, e.g., when Mike
is at the machine, Tom can't take over and mess him up.)  You will need to 
change the Default password, which you access through the shortcut: 

   Start -> Programs -> VNC-> VNC Server -> Show Default Settings.

Another perspective from rich_at_nospam-darkage.co.uk
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As mentioned in a previous FAQ - Windows NT, 2000 and XP can have different
passwords for each users. If you want to remotely admin the PC with one 
single password you need to change this for the "default settings".

Solutions:

1) Drop into DOS and CD to the folder containing VNC (usually 
   program files \ orl \ vnc). Type winvnc -defaultsettings and press [enter]. 
   A GUI box will appear and you can now change the password for all users. 
   Now restart the VNC service (which will disconnect you if you're using it!)

2) Use REGEDIT and connect to the remote computer. Look for the key called: 
   HKLM\Software\ORL\Winvnc3\default. Change the encoded password to one from 
   a box that you know works (nb: you could delete any user account entries 
   that you don't want).

3) Use one of the vnc password tools and Xcmd / PsExec to change the password
   remotely.

4) Use the Fastpush script from www.darkage.co.uk :-)