Back up these:
1. The entire incomplete folder. (You could leave out those files at 0 KB if you wished. The most important files in this folder are the download.dat & .bak files. These are your downld preferences. So safest if you backup the entire folder.
2. Optional: The LW Preferences folder called .limewire & found here: C:/Documents And Settings/your_username/.limewire OR Windows 98 / ME: Your .limewire folder is in "C:\My Documents"

Backing up the LW prefs folder would simply make it easier & no need to try to remember your previous settings. By the way, after you re-install LW, you should replace the incomplete folder with the older backed up one (whilst LW is closed.)