Re: Run Phone line through PC
VOIP? Why didn't you say so?
It depends how much money you want to throw at it...
Simplest/cheapest single-line solution: Sign up for Vonage, Broadvoice, or another similar service, plug an analog phone into the adapter and go. You can go with multiple lines (usually two per adapter), but you won't have the advanced features of a PBX.
Most complex, but sounds like what you really want: Install an asterisk based pbx software package onto a computer (Or a virtual machine), buy voip phone service (Broadvoice will provide service for your equipment... there are other providers, but Junction Networks is the only other one I have personal experience with) setup several VOIP phones (or VOIP adapters connected to analog phones) and configure. The PBX system that I've used the most (and the one that the company I work for resells) is Switchvox... there's a free version available for download.
One of the biggest things to look out for with internet based phone service is that you NEED QoS on whatever router you're using for your internet connection... ESPECIALLY with multiple lines. Otherwise you'll lose phone quality when your connection sees heavy usage... lag, sound breaking up, etc.