Try deleting LW's preferences folder when LW's properly closed. See point 3 here & also point 2 if you've used older versions of LW in the past (there could be a folder in either location).
Fixes for LimeWire
This will change all of your settings within options, you'll have to go back through afterwards & reset things. If you're not sure of your settings, go through LW's tools > options & write them all down, so that you can reset things later.
In case you're using Vista: http://www.gnutellaforums.com/genera...der-vista.html

See whether updating Java helps. You can either try updating to the latest version here (I have this one & LW's working fine): Java SE Downloads
or try this older version, which has worked well for lots of people: Archive: Download Java 2 Platform Standard Edition (J2SE) 5.0 Update 11
Whichever Java version you use, download the JRE offline installer & remove any other Java versions before installing another one.

If those fixes don't work, update your AV & scan your computer. If you don't have an AV, try one of these free programs. Make sure to update before scanning.
avast!

AVG free

Avira Antivir

Do you scan files with an AV before opening them? Had you just opened a downloaded file when the problem started? Are all of your saved files still playing properly?

If you have the 4.12.3 installer saved, you could try uninstalling LW (start > all programs > limewire > uninstall) & reinstalling. If you don't have that Pro installer saved, you'll lose that version of LW if you install another version.