Do you see the error with every file that you try to download? How far does the download progress, before you see the error? Does it happen with all file types?

Which version of LW are you using (5.1.1 or 5.1.2)?

Had you used LW 4, or earlier versions of LW 5 previously with no problems?

Which version of Java do you have installed?

Try either reinstalling the latest version of Java or rolling back to an earlier version. Download the JRE offline installer & remove any other Java versions installed (via Control Panel) before you install another version.
Latest Java: Java SE Downloads - Sun Developer Network (SDN)
Earlier Java (JRE 6u5): Archive: Download Java Platform Standard Edition (Java SE) 6u5

If that doesn't help, 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
LimeWire Pref Folder in Vista
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.

And if nothing helps, scan your computer just in case you're infected. LW 4 users had some weird issues with options when infected. Just in case, check out the advice here (give AVG a try):http://www.gnutellaforums.com/genera...tml#post310134