Make sure LW is shut completely down(not running in the taskbar). Make sure you have your folder options set to show hidden files and folders, Tools>Folder Options>View, tick by "Show Hidden Files and Folders", click apply.
Then go to C:\Documents and Settings\"your account name"\Application Data\Limewire. Delete the folder called "LimeWire". It should be there.
Unless your using
Vista. See this thread:
LimeWire Pref Folder in Vista