It might be a virus. Does it start up by itself after you shut it down? If so, there is a worm that causes that.
Try the fix in this thread:
To fix the limewire keeps popping up / no taskmanager bug do the following
More virus info in this thread:
WARNING: Viruses on network you should be aware of!
Here are some sites that offer virus help:
http://www.spywarewarrior.com/index.php http://www.castlecops.com/forums.html http://forums.spywareinfo.com/index.php?showtopic=79038
You could try uninstalling LW and see how your computer runs without it. Uninstall it by going to C:\Program Files\LimeWire\Uninstall.exe, double click the uninstaller. Don't uninstall using the add/remove programs. See how that goes. If an error message starts popping up then it's probably a virus.