Damn, Sorry, I totally looked over this thread!
You need a multitrack recording program... so you can record midi tracks (with an additional midi keyboard, not essential, though), and audio tracks. You could sing through your pc-mic, should be "reasonable" quality.
Some multitrack programs:
-Cakewalk Pro Audio (I use this, and am pleased with it)
-Steinberg Cubase VST or Cubase SX if you have windows xp
-Sonic Foundry Acid (not sure if this supports midi nowadays)
-maybe some Magix software....
Some audio editing programs:
-Syntrylium Cool Edit Pro
-Steinberg Wavelab
-Sonic Foundry Soundforge
With the exeption of Magix, these are all programs used by professionals, meaning it's a little harder to get started. When you do take the effort however, your music or songs will sound much better.
If you get hold of one of these programs, it might be handy to buy a full duplex soundcard. Maybe your card is full duplex, you can check this by viewing its properties. A new, simple soundblastrer card costs about 30 bucks. A full duplex card enables you to playback and record simultaneously, so for instance you can hear your piano track when you record vocals.
hope this helps!