I make more mistakes than anyone else around here (it's in the rules) so you'll do just fine. The first thing I'd advise is making iTunes the default format for downloading from the internet (it's very inconvenient to play MP3 files using Quicktime). Next, a word of reassurance. The chances of a virus affecting your machine are almost nonexistent. The nasty people who devise these things aim them at the gaping holes in the Microsoft operating systems and packages.

Now your question. Sometimes a file will not launch from Limewire because it has been corrupted. Go to iTunes and use Add to Library. If that does not work, go to the Shared file and double click on the file itself. If it is a valid MP3 file, it should play in whichever application claims it (whether iTunes or Quicktime). If this direct clicking fails, the bad news is that you've picked up a dead file (record companies or other malicious people do post spoof files out there to make life difficult). Do not be sentimental. Trash it and try downloading it again.