How do I change the login screen in Windows Vista to show the clock and world clock?
Have a look at the following tutorial http://www.howtogeek.com/howto/windows-vista/customize-your-windows-vista-logon-screen/
I assume you want to see a clock when you have your account locked and you're at the login screen?
Since in that case I think the only solution is to install a clock screensaver and make it On resume, display logon screen
That way you're computer will show whichever clock screensaver you can find and will return you to the login screen if you have to logon again.
Note: I understand this isn't what you're asking for. But AFAIK you can only change the background image of your logon screen and I don't think there exist any dynamic wallpapers that would allow you to have a clock on it...