Well, some users experienced problems after updating due to their AV, most of them with Norton Antivirus.
Some had problems because they used the online installer instead the offline installer.
Did you try to delete your LimeWire preference folder (which should be located in the hidden Application Folder under "C:\Documents and Settings\Administrator.<your computer name>\Application Data\LimeWire"
Delete it with LW completely closed (i.e. also in the system tray). If you updated from an version earlier than 4.12 then you should also try to delete the " .limewire " preference folder of the old version.
