Files Not Recognized

When I try adding some files to the iTunes music library, some are not recognized. There is no error message or prompt, they are simply ignored. The files are un-encrypted in mp3 format and they play in WinAmp and Windows Media. This does not happen with all files, only centain seemingly random ones.

I am using iTunes 6.0.2 and running XP SP2.

I've tried adding by folder, file, and by dragging file but nothing seems to work. Thanks in advance.