How serious are you about this ?
It can be done but needs processing power at the 'nodes' so that it will not be a one machine solution (but to expect a multi source whole home video distribution system out of one machine is too much IMHO)is a no no...
Small form factor machines are common now... With hibernate features yuu can have very fast boot times...
Snapstream and SageTV can IIRC do multiple streams throughout a home (as does the less elegant but cheaper VideoLan)... As I understand it Snapstream looks better but Sage may Sage have the edge in under the hood power...
Certainly running multiple input streams to output nodes... Performing PVR work scheduling from the nodes for the server to process... Feeding those streams with the usual PVR abilities to the nodes... These things can be done... They are however pretty cutting edge for PC based whole home video distro and being on the bleeding edge can be painful if what you are trying to do is set up a family friendly system...
I would say that unless you are very familiar with the topic or are doing this for the fun of it, pay for a reliable quote on a standard video distribution network from a Home Automation installer, your family will thank you (as will the custom installers whose son you probably put through college
)... And thats coming from one of those early adopters !!!