Hi dbulfer
I use MCE2005 on my HTPC and stream all my mp3's, photo's, ripped dvd's and High Def movies from my NAS and haven't suffered any problems with the pc slowing down, I've played a couple of HD movies right through this week so maybe 2 hours of continual streaming and my HTPC hasn't noticeably slowed.
MCE2005 is a cut down version of Win xp, there are various opinions as to whether its a beefed up Home edition or a cut down Pro edition but it is win xp with the media centre bit added on, I would be surprised if Bill and co had used a different version of CIFS for his "value added" product. But this is the world of microsoft so nothing should be ruled out
I'd be considering re-installing MCE first and then checking network cards and cables. Problems with NASlite are 99.9% of the time hardware related and not the actual software.
Good luck