Song purchased from iTunes Store won't play on my computer

Question

Several people have posted variations on this question but I haven’t been able to find a coherent explanation of, or fix for, the problem.
I have downloaded all my CDs into iTunes and synched them to my iPod. Everything downloaded from CDs plays fine.
I bought and downloaded one song from the iTunes store. It appears on playlists, but I cannot play it using iTunes. When I click on it I get the blue highlighting but nothing happens when I click on the “Play” arrow. (I can play it using Quick Time). My computer is authorized.
When I synch the iPod, I get the error message “(song) was not copied to the iPod (my iPod) because it cannot be played on the iPod.”
I noticed that the files copied from CDs are .m4a files and the file downloaded from the iTunes store that won’t play is an .m4p.
I think this is some kind of rights issue. Can anyone explain what the problem is? Thanks in advance.
Dell   Windows XP  

Answer 1

The trick of deleting the song to the Recycle Bin, dragging it to the Desktop and then back into iTunes worked. My guess is that because our computer was not authorized at the time of the purchase (this was our first purchase from the iTunes Store), the original file downloaded in a protected format. The computer has since been authorized, so the Recycle Bin ruse tricks iTunes into thinking it's a new file. I would still like to hear a confirmation of this from someone who knows though. Also, Apple should tell people to authorize their computers before making any purchases. It will save some confusion it seems.
Thanks for the tip.