I Want My (Digital) Rights!
It does seem unfair... you paid for those songs and now you can't play them outside of the iTunes environment, on hardware not supported by Apple, or on operating systems not supported by iTunes.
That's because the iTunes tracks that you purchase and download are in a "protected" AAC or M4P audio format. Only it's not YOU that is being protected, it's Apple, or rather Apple's bottom line. Apple explains this by saying that their proprietary format gives you superior sound quality, but that's a smokescreen. It's all about DRM, and DRM is all about the money.
The ability to make copies of copyrighted materials is seen by the recording industry as a threat to its profitability, and perhaps rightly so. DRM (Digital Rights Management) was created by digital media publishers so they could control the duplication and dissemination of their content. But DRM is actually a misnomer. Because rather than granting rights, DRM actually restricts the consumer from doing perfectly valid and reasonable things with music they own.
MP3 is the standard for digital audio. An MP3 music track can be played on almost ANY player, whether it's portable or computer-based. You can burn MP3s to a CD and they'll play just fine on almost any modern CD player.
But Apple's iTunes software doesn't create MP3 files when you buy a song. The files are "protected" and cannot be played on a computer which does not have the iTunes software. You CAN copy them to your portable music player, as long as you bought that player from Apple and it says iPod on it. Understandably, this makes Linux users, and the millions of owners of non-iPod music players a little upset.
Converting iTunes to MP3
If you right-click on a music track in iTunes, there is an option to Convert Selection to some other format.
Probably it says "Convert Selection to AAC", which is useless because your iTunes tracks are already in AAC format!
So click on Edit / Preferences / Importing (or Edit / Preferences / Advanced / Importing) then change the Import Using from the default setting to "MP3 Encoder". Click OK to save this setting.
Now when you right-click on a music track, there is an option to Convert Selection to MP3. But just try it... iTunes will tell you that "Protected files cannot be converted to other formats."
Converting AAC to MP3
Bzzzt! Sorry, thanks for playing. It turns out you can only convert non-protected formats (such as WAV or WMA) to MP3. So what can we do? Fortunately, there are two ways to get around this annoying restriction.
Method 1: Burn the track(s) to a CD, then you can open the CD in iTunes and the Convert Selection to MP3 will do exactly what you want. Just be aware that you'll have TWO copies of that song in iTunes -- one in the original AAC format and one in MP3 format. You can delete the AAC version if you like. (Note that you must burn an audio CD, not a data CD. Check the setting in Edit > Preferences > Advanced > Burning if you have a problem importing tracks from the CD.)
Method 2: If you have lots of music, you might need a big pile of CDs to convert everything with the "burn and rip" method above. That's where the JHymn software can help. JHymn was created to allow you to exercise your fair-use rights under copyright law, and will free your iTunes music from DRM restrictions with no loss of sound quality. JHymn provides a drag and drop graphical interface for both Mac OSX and Windows users. But currently, JHymn will not work with iTunes Version 6 or higher. If you have an earlier version of iTunes, keep it!
Note that JHymn is not meant to aid music piracy, but it does contradict the iTunes user agreement, and may not be legal in your country. You should be aware of the legalites of DRM circumvention in your country and make your own decision whether using JHymn software is right for you. Then use JHymn only for making archival copies of your own music, for copying tracks to an MP3 player, or for playing your music on a non-Itunes platform.
Along the same lines, the ImTOO Video to Audio Converter can convert AAC files to MP3. But since there is no legal AAC decoder, this program operates under the same legal cloud as JHymn.
For more information on the legalities of DRM circumvention, see
http://hymn-project.org/jhymndoc/jhymn_faq.php#legal