Which version of LW 5 are you using?

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: LimeWire Pref Folder in Vista

If you want to try a reinstall, go through steps 1 - 8 here: http://www.gnutellaforums.com/333278-post2.html
Instead of using the LW download link there (it's a link to basic LW), download from your Pro download page If your Pro benefits have expired & you don't have your Pro installer saved - you'll lose your Pro version if you download a free basic LW.

Does it happen with every file that you try to download (even really old songs)?

I haven't ever seen this message & only a couple of people have posted about it eg: http://www.gnutellaforums.com/downlo...r-message.html

I'll see if I can find out whether that error message equates to some error messages in LW 4, or points to a specific problem.