Yea, um, Oneironaut is right. Unfortunately, the average consumer has no clue as to what is going on as far as the real differences between OS X and Windows. Then there are the avid PC users that claim there are viruses for OS X and/or the "small market share" argument, which is also baseless. Also, 3rd party PC stores... I was spoken to at Best Buy for telling somebody that there are no viruses for OS X. I ended up winning the argument (heart proven sources), but these guys want to push sales, so they'll still try to get you to buy antivirus for Macs and all that good stuff. This just leads consumers more and more to not know or not believe.
Anyway to the OP's actual problem. I've had this happen a couple of times, and it can happen for various reasons. Several I've encountered:
-You have changed the location of your iTunes library and for whatever reason, iTunes is looking in the old place (maybe you forgot to do it in iTunes itself, or it just glitched and is still looking in the old spot for some particular songs, which has also happened to me)
-You may have moved/renamed a file or 2 in the finder without remembering. Use spotlight to try and find the song when it searches your HDD. If you have automatic sorting and organizing turned on in the prefs, it will reposition the song in relation to its ID3 tag.
-You have an external hard drive that you keep your iTunes library stored on, and you opened iTunes before mounting the drive.