MCE isn't usually too stable on a machine that wasn't designed for it. Your best bet is to build a machine with XP, and use some good software like MyHTPC or BeyondTV.
The only legal way to obtain a copy of Windows MCE is with a pre-built computer that is already equiped with the OS. I know some HP's come with it, i believe some of the Toshiba's do, and now i guess Gateway has a Media Center PC out as well. Kind of sucks, because it is actually a pretty sweet setup for an HTPC. Also, if some how were able to get a copy of the MCE and put it on your own computer, it won't most likely be compatable with your TV Tuner card. There are only a few cards that are compatable (i'm unsure which ones). Leadtek recently came out with an MCE card, but i haven't tested one out.