If I bought a CD and ripped it to my PC, then sold the CD, is it illegal to still keep the songs?

if you sold the cd yes, because by selling the cd you are selling your rights to those songs. why did ms create the rip feature? as a back up, so you could make a backup of something you own, allowing you to use the copy and keep the original from being damaged..which is one of the reasons they created cette tapes.