Re: line in??
Your PC has a Line In built in - it's a part of your soundcard. Granted those built into a motherboard might not be as good as a PCI (internal add-in) card, but modern motherboards are improving in that department from what I've seen lately.
It's a stereo 1/8" jack like you'd find on a set of headphones/earbuds (iPod, etc).
But there are also things like have been mentioned by Line6 and m-audio that work through USB and/or Firewire ports.
However, I'm not convinced of their reliability on "all-in-one/general purpose" PCs. Yeah if you've got a PC that does nothing but music, that's fine, but if it's cluttered up with external hard drives, anti-virus software, games, ICQ, and other programs that bog down the system, you'd be better off using a dedicated Line In.