1) Plug your headphones/earbuds/earplugs into your mic jack on the CPU.
2) Go to Start->Control Panel->Sounds, Speech and Audio Devices/ Sounds and Audio Devices. Now under the audio tab, make sure that the sound recording setting is set to your sound card. For best results turn the volume of the sound recording up to full. Similarly go to the voice tab now and change the voice recording in the same way.
3) Open the sound recorder, Start->All Programs->Accessories->Entertainme
4) Just press the record button and start recording your masterpiece.
Note that depending on the brand of headphones/earbuds/earplugs that you are using, you may have to put them fairly closer to your mouth to record the voice properly.