There's probably going to be a variety of programs suggested, but it's not so much the program you use, it's the bit rate you convert to. Windows Media player will work just fine. The higher the bit rate you convert to, the wider the frequency range you'll get out of the audio. For most listening purposes, 256 bit is fine. WMP converts to MP3's up to 320 bits.
AAC, in general, will have better quality at lower bit rates, but depending on what you're listening through, you're not going to notice a difference. I prefer to use MP3's. I'm guessing you're just looking to listen to your music at an acceptable quality and not really concerned if you can't hear the frequencies about 20kHz. Unless you're looking for a better media player, I wouldn't worry about it.
