Could have several reasons......besides you haven't answered some essential questions possibly necessary to give you some proper advice...
If you try to download files with hundreds of hosts, forget about it, since those are most likely faked files. Try to choose from files with 10-20 hosts.
When downloading and installing Java, did you use the JRE offline installer? And before installing it, did you remove all other Java version from your computer??
One thing to do, if all the above failed is to delete the LW preference folder. Depending on your version (4.12. or older) they are named " LimeWire" and " .limewire ". The first one is located in the hidden Application folder under C:\Documents and Settings\Administrator<Your computer name>\Application Data.
To find the latter one, follow the link here. But make sure that in either case (or both, if you had an older LW version previously installed) LW is completely closed, i.e. also in the system tray (Point #2 & 3 in Fixes for Limewire)
http://www.gnutellaforums.com/showth...threadid=36420 