Cant believe you are looking to replace Roku...
Let me add my 2 cents as well to this discussion...
I am a nerd with 3 PC's at home for blueray, hypervisors and gaming, I love W7 though its just not good enough to be operated with dedicated remote which can allow to play on XMBC/boxee and media center, there are lots of customization and headache you need to go through to make things work and then there are lots of updates.... and finally in the end your experience is not as smooth as Roku XS (blue ray support is phethetic with windows media center softwares such as WMC/Boxee/XMBC)
I have tried very hard (quite an advance user and automation expert) to make it work though in the end its too much hassle and not the best experience...
Another amazing plus with Roku is 2-3 watts of power consumption only, amazing considering electrivity bill can easily add around 10 bucks/month for your PC...and remember ROKU is dead silent
Netflix/Huluplus have the best experience anyday compared to any other software/media player.
Also my experience with XS (wired networking) is the best and no issues/delays noticed. does USB MP3 playback very nice and MYmedia channel allows to play everything (photo/video/music) from my PC, this experience is very very smooth (caveat MP4 video is only supported, i guess MKV support is there as well though didnt work for me)
Now experience of streaming channels and selection is just amazing, other than usual if you add provate channels (nowhere, though it hung sometimes for a min, not while playing but while navigating) you are able to get several latest podcasts and TV channels
I got my first Roku 2 weeks back and I am realizing how much time I had wasted trying to build a perfect media center PC, WMC is still not matured in my opinion, lots of hacks though functionality is just not increasing within software...
And none in your family is gonna like PC anyway due to cumbersome experience, while Roku always get WAF easily and always on....
I ordered my another Rokuy XD for another room today as there is a daily deal on amazon...
I configured one of my PC with win 2008 R2 as DC/NAS/ISCSI/MyMedia so that in one box I have all media to share, Roku is able to see contents easily, though on MP4 video's are supported.... (i tried converted MP4 from blueray movies i had for Iphone and they worked awesome, amazing amout of clarity on 40" screen)
No media center player as of now plays Blue Ray ISO's the way my popcorn A200 does (again lots of customization and haggling to make it work in MCE and XMBC, as no-one support as default and you have to pay for blue ray software as well)
You will be able to play all type of media's in PC though thats about it, other than that ROKU wins in every department....
Now here comes the roku's shortcoming...
Remote is limited, i like to see a "next video" button included when playing playlist of streamed shows
no sleep timer default, nowhere channel does support it though (it saves bandwidth when you sleep while playing a channel playlist)
XS model has only RF(which is a plus) but not able to configure in any universal remote, newly ordered XD has IR thus will be able to have one remote setup now...
No Youtube (playon software allows that though streaming expoerience is not good at all), there are talks youtube is coming on roku soon
WD live doesn't work as it does not have much of streaming channels (NBC/ABC/CNN/NHL/NFL) as all I prefer to see in morning is latest selected news...
Sorry for any typo's /spellings above