What can I use to improve the quality of CD’s and MP3’s that I play on my PC?
You can not “improve” existing files. Once compression has been applied, audio data is lost forever. That being said, Creative’s X-Fi sound cards are quite impressive at being able to abstract the lost information and restore sound fidelity. External processors for mp3 players and the like are also available.
If you are talking about improving files that you rip in the future, you need to change the encoding settings. Ideally, you would use a lossless codec like .ape or flac.
If you are talking about the sound coming out of your PC then a good set of quality speakers and a high quality soundcard will help a lot.
Also good Sound Card has a lot to do with playing quality song in PC,