I’ve successfully upgraded a not-so-old PC to Windows XP Media Cetner Edition 2005, and so far it’s going great. I started with the Designed for Windows XP Media Center Edition 2005 Logo Partner List, picking up an OEM copy of Windows MCE 2005 (naturally), a Hauppauge WinTV-PVR-250MCE capture card, Nvidia’s DVD Decoder
(absolutely required – without it, you won’t be able to display a TV signal), and a Microsoft MCE Remote Control.
I had to download the latest MCE drivers for both my Nvidia video adapter and the Hauppauge card. After updating those drivers and installing the DVD decoder, everything just worked. I’m very, very impressed by the interface, the quality of the picture, the general reliability, and the speed of operation.
Next step: Installing a Media Center Extender so I can play music and watch recorded programs on the big screen in the den.
Ed Bott 
Where does one, ahem, acquire a copy XP MCE?
OEM copies are now available from most dealers who sell Windows. I recommend either Newegg.com or PC Alchemy. The latter has some excellent MCE bundles at attractive prices.