Reduce shares & incomplete files if you upgraded from LW 4. LW 5 does not handle too many of them very well. At least reduce shares initially. Try changing compatibility mode, see points #5 & 6 here: Fixes for Limewire (click on blue link)

You might also want to try point #3 about deleting your LW preferences folder if the problem persists. If still problems, try point #1 about removing Java, reinstalling it. Here is one approach to reinstalling http://www.gnutellaforums.com/333278-post2.html