The preferences folder is found here: C:\Documents and Settings\*yourusername*\Application Data\limewire
if you started using LW less than 2 years ago. The App Data folder is Hidden so you need to show all hidden files & folders in your search options before searching. Example of how to find at bottom of this thread Fixes for Limewire (click on blue link).
However, copying your own prefs folder may present some issues. Their LW might not open, giving permissions error because the new download directory will be in your account. So I am not sure if this is really a fast way of doing it or not, & might be more trouble than it's worth. I'm not sure of how else to do it. You could perhaps for the time being set your own LW prefs to download to a general shared folder which is accessible to all account users. Then simply change each one in each account. And after that, change yours back. To make a folder accessible to all users, you need to set that in control panel, I think it's the admin one 'off-hand'.