If you record directly into the computer there's a good chance you'll get the background noise of the computer as well. If you have somewhere else and something else you can record onto, as the Guru says, quiet and without echo, it will be better. I record onto my camcorder, for want of anything else. Advice I was given for reducing echo was 'talk into a curtain' so I do - in the bedroom, sat on the quilt facing the window, curtains closed. I use a separate directional mike, not the cams onboard, and keep it slightly to one side of my mouth, so I'm not breathing into it.
That article says most of it, but I also attended a voice over workshop and the actress who gave it, said to do warm up exercises, both arm and shoulder movements and saying tongue twisters.
Soundforge has ways of adding echo and reverb, for different effects, experiment. If you do have any electronic hiss or motor noise, Audition and the free one Goldwave have noise reduction filters, but you need to try to have as little hiss as possible, as the filters do affect the voice quality slightly.
Win XP, Athlon 1.8, G force 3 T, 1 60gig drive + 1 250 gig, Pinnacle Studio 9 plus mostly, Premiere 6, and Vegas 5 sometimes, Soundforge, Cool Edit.