As long as you know the e-mail you used to sign up with and 6 months hasn't passed since you purchased it. You should be able to download it by putting in your e-mail address here:
http://sales.limewire.com/support/pro_lookup.php
If your past the 6 months, you'll either have to purchase it again or download LW Basic(free)
.
http://www.limewire.com/english/content/download.shtml