WarningThis no longer works
Open the control panel and if you have the consumer version you will see add features to Technical Preview.
Click this and it will ask for a key.
Use this key MBFBV-W3DP2-2MVKN-PJCQD-KKTF7 and accept the licence terms. Some grinding off hard drives a reboot and there you go media center.
You will know you have the media center as it states 8.1 with media center at the bottom.
PLEASE BEAR IN MIND THIS IS ALPHA SOFTWARE AND IT MAY HOSE YOUR SYSTEM.
You will find the icon in Windows Accessories. Right click and add to taskbar.