Susan Bradley, the Microsoft MVP for Small Business Server, has been on a bit of a rampage this month about OEM's selling businesses machines with "Windows XP Home Edition" intalled on them. Later, when they try to join their "XP Home" boxen to a domain, they find they can't. Since I get asked this on a regular basis as well, I'm linking to it so that Google will bump up the PageRank.
"Windows XP Home Edition" CANNOT JOIN A DOMAIN!
And remember, even if you do join an XP MCE 2005 box to a domain, there are several things you can no longer do with it, such as use Media Center Extenders.