First of all, don't listen to all these guys who diss you. They are just bitter because they spent a lot of time and money on expensive equipment before companies like Radio Shack started making equipment equal to or exceeding that which is found in pro studios.
Realistic mics are excellent for recording, and the reason you are getting a muffled sound is not because of the mic itself, but how you are positioning it in front of your face. What the pros DON'T tell you is that because of its unique frequency characteristics, you have to hold it to your nose, and NOT your mouth.
Once you do this, you will get a much better sound, but it still won't be studio quality. That's because the difference in what you track and what you hear on a professional recording is always done in the mixing stage. And you are in luck: Realistic makes some GREAT equalizers. Just get a stereo Y cable, plug one end into your stereo line-out of your pc and into your equalizer. The output of your equalizer (using a similar cable) goes into the mic input of your soundblaster (or whatever your soundcard is. Soundblaster is the soundcard pros use, but if you have something cheaper, hopefully it will be as good).
Boost the highs where necessary, and don't forget to use the "stereo expander" on the unit to separate your mix as hard left and right as possible.