Just because you found on one occasion that a file would not launch is no reason to delete it. Obviously, there are some false files out there in the cyberworld that contain spam or worse, but sometimes perfectly playable files will not launch directly from either the download window or the library window. Before moving to trash a file you should:

(a) go to iTunes and try the Add To Library routine. If that fails,

(b) open your Limewire Shared folder and Get Info on the offending file. Assuming it is an MP3 file, double clicking on it should launch it to your default player. Check in the Get Info dialog routine for the application nominated to open the file.

If you want to be more proactive, there are shareware products designed to modify the formats of files but there are certain risks associated with their use if you are not confident of the true nature of the file contents.

Only if all this fails should you trash the relevant file and waste your effort in having found and downloaded the file.