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.

__________________
If it ain't broke, fix it till it is!