Ok, you can to buy the O.S. as OEM by it's self, at most pc stores.
I'm from Australia, but I did a quick search at newegg.com and they sell it there, lots of other places will too. You'll also need to buy the mce remote, forget about using your tv tuner's one.
In Oz, I'd pay about $150-170 for the O.S. and about $55 for the remote, so those prices look alright if you're in the U.S. I'd look through ebay too, you might pickup a good bargain (just watch out for those cheap MCE imatation remotes, get the real MS one).
You'll need a video card that does directx 9 and tv tuner that has bda drivers and a hardware mpeg2 encoder onboard. Building an MCE machine isn't pain free, but it's worth it.
Before you buy anything, read through some of these sites.
P.S. you might want to have a look through the official MS MCE site too, and grab one of their sexy new MCE keyboards with onboard mouse when they come out very soon too. much better than a normal wireless Kb. http://www.microsoft.com/windowsxp/mediacenter/default.mspx
#ad_buddies Afterdawn chat channel.
Server: irc.stormchat.org Channel: #ad_buddies
Have mIRC or IRC Client? copy/paste this link:
"irc://irc.stormchat.org/ad_buddies"
No mIRC? use the Webchat: http://www.adbuddies.org
Looks like the price has risen a bit since I last looked, $190 seems to be the going rate in Australia. I did see these two stores have it for $177 & $179 respectively.
#ad_buddies Afterdawn chat channel.
Server: irc.stormchat.org Channel: #ad_buddies
Have mIRC or IRC Client? copy/paste this link:
"irc://irc.stormchat.org/ad_buddies"
No mIRC? use the Webchat: http://www.adbuddies.org