What can I do to produce perfect vocal harmoizations?

I write songs using an acoustic guitar and vocals and record them at home. I use a usb condenser mic on either Audacity or Cool Edit Pro. Is there anything I can do to produce perfect back up harmonizations using just a USB mic and these simple programs? I normally just sing them on a seperate track, but I was wondering if there are any programs or VST plugins that will create the harmonizing vocals for me...

Also, is there any way I can do this in real-time? Like using a pedal and amp, only through my PC. I know they make a pedal that can do this, is there a way I can do it on my computer?