If you want to convert songs you have bought from the itunes store to mp3 do this:
Open iTunes
Make a playlist of all your bought songs
Burn them to disc
Open Windows media player
Press the Alt Button
Click on Tools
Then Options
Then select the rip music tab
Then on the drop down menu next to format select MP3
If you want to convert the songs on ur itunes that you have encoded from your own CDs you can do this:
Open Itunes
Click on Edit Then Preferences
Then click on the advanced tab at the top
Then on the tabs belw click Importing
Click on Import Using
Then select MP3 Encoder
Then select your file quality