MediaPortal runs on XP and makes use of standard codecs, sql express/mysql and .net to make things work. Happily streams live video to multiple clients and a vlc plugin allows you to tap into the stream using that app. All that's missing is a web frontend for doing everything the fat client can.
They just launched their first full release and have version 2 on the go already, plenty of plugins to add extra functionality.
I've tried many setups - trials of beyondtv, linuxMCE, knoppmyth, mythbuntu etc and none of them behave the way I want them to or have spurious problems that apparently should never happen.
MediaPortal certainly has its' own bugs, but on the whole it tends to be pretty solid and operates in a server/client config that suites me :)
Using apps like autogk you can re-encode the recordings from the mpeg2 into smaller mp4 based formats and then file them away on your nas box.
HDD storage is cheap as hell nowadays - £70 for a 1TB internal HDD, yummy :D
I run the TV service on my main rig (Dual core goodness means I can play left4dead while the TV server records something for me) and have a bunch of clients based on toshiba sat pro L20's (with their XP keys on the bottom for legality) that I have salvaged from work due to the hinges snapping. They have svideo and vga out so I can make use of their small form factor and better energy consumption rates compared to a desktop.
Easy to move around the house should I wish to re-jig or repair things, use a PDA or an iphone to access the ipimp plugin so I can remote control the mediaportal client and schedule recordings without having to fire up a client.
The L20's even have competent DVD drives so you can use the DVD player in mediaportal to watch them should you feel the need.
Just need to buy new monitors for bigger viewing :)