Have you tried downloading a file with a single modem source? What happens then?

Deleting the preferences folder has been a fix for those people who suddenly find that literally everything they try to download needs more sources. If you're going to delete the prefs folder, don't forget to look for one in the new location of C:\Document and settings\your username\Application Data\LimeWire (hidden by default). The .limewire folder is the old prefs location, for LW versions older than 4.9.34.