If it's only starting up automatically once (ie when you first start your computer), try deleting the preferences folder when LW's closed. See points 2 & 3 here
Fixes for LimeWire
This will kill all of your settings within tools > options. You'll have to go back & reset things afterwards.

If your LW starts up automatically all the time (ie won't stay closed), it's a virus
WARNING: Viruses on the network you should be aware of
In this case you should follow the link to Norbiesworld within that thread & register for free computer help!