Hi, I'm a first time user on here so please be patient with me if I'm posting in the wrong place! I have windows Vista and AVG, so no virus. I'll download a song from limewire, then one day when I go to play it, it has an exclamation mark and an error box says "file can not be located". Help! I'm slowly losing all my good songs, one by one! This has never happened before and is VERY frustrating! Thanks to whoever can give me advice!

It means the songs are not there. Where are you playing them from? iTunes or another audio program? If iTunes then add them to the iTunes music library as described in a sticky at top of this section. How to transfer music to my iPod (& iTunes) {click on link}
If from LW, then it means you are for some reason deleting or moving your songs from the LW download folder.

Playlists are a document that notes the location of a file & points a player to where to find it. If you add a file to a playlist & later move or delete that file, then the playlist won't be able to find it & have similar result as to above.