We have an open discussion about it but the one thing I learned is that some didn't like others not giving the correct answer. Please accept that there are multiple causes.
My most common cause is Windows Media Player's setting of "rearrange and rename my music." The second cause is limewire and such software.
Ok so first of all I'm using iTunes 8, and a 1GB Sansa C240 MP3 player, and I'm using Windows XP SP2.
The Problem: Ok so I probably have somewhere over two hundred songs on my iTunes music library right now. The problem is is that for a while now, iTunes has been losing the original file locations for multiple random songs (they'll have a gray "!" next to it). It does this to random songs and at random times, and it's really annoying cause I have to click "Get Info" and it will say something like: "Could not be used because the original file could not be found. Would you like to locate it?", and I click yes, and I find the song, double-click it, and it's in. It's not a big deal, but when I have to do it for over five different songs, it can be a big pain cause it sometimes deletes the album art, too, and it just takes a long time to go through each one and re-locate it for iTunes. I locate all of my songs for iTunes from a folder I keep all my MP3 songs in, by the way.