Sorry for slow response, I've been away.

Need to make sure the files you are downloading are not some kind of spammed file. Check the file sizes ... Virus thread: Typical virus spam file sizes - Make a note of them (click on blue link to see). Also, check this one mp3 Bit-Rates Versus File Sizes (click on link).

Keep in mind iTunes cannot play all music formats such as mp3pro, ogg vorbis & some others. Get a back up player to double-check those files: http://www.gnutellaforums.com/291443-post3.html such as with ToolPlayer.