Media Center is only available as an OEM (original equipment manufacturer) install. It is a version of the XP OS. If your PC didn't come with it from a place like Dell you can't normally upgrade to it.
But if you like living dangerously, see Convert XP To MCE 2005 :
I stumbled across a conversion kit for
XP Pro to MCE.
The full Blog can
be found here.
link to the ISO file can be found
Password for the zip
file is : beefteck
reading the entire blog, and
comments. Some had no problems
booting from the disk for the registry
hack, but others like myself were not
able to boot off the cd.
registry modification is very simple,
and MUST be done BEFORE you attempt
the conversion, but it CANNOT be done
in Windows, as the keys involved are
protected. I used ERD commander,
but BartPE should work also, as both
allow access to the Windows
So far, I've used it on
a new Windows XP install, and I also
converted my existing XP Pro SP3 ASUS
OEM install. The great part is
you don't have to hack or crack your
existing installation for Validation.
After the conversion, my ASUS OEM SLP1
install is still activated, and passes
Windows Genuine Advantage. All MCE
functions are working perfectly.
As always, you should backup you
current system before attempting this.
Do not rely on System Restore for
something like this.
You can also google for "XP to MCE conversion torrent", but you might end up installing a virus.
I have no idea how legal this operation is, but I suspect not very much. If the above article is valid, make first a disk image of your system drive to avoid having to reinstall Windows if things go "Boom!". I take no responsibility.