That is not what he said, although the Gate part may be confusing.
Microsoft requires all PC manufacturers to provide some way to restore a PC
to the Factory installation when an OEM version of the OS is installed.
This can be an OEM XP CD, system restore cd or a hidden partition on the
hard disk drive. Some, but not all, manufacturers also provide a way to
burn a restore CD from that hidden partition.
To return to the XP Media Center edition, you will be required to reformat
(either by using the bootable system restore disk, the OEM XP MCE disk or
following the instructions on the hidden partition install method) the
partition of the hard drive where Windows OS is stored and reinstall the
Media Center edition from wherever it is currently stored. Read the
system's documentation to find out the manufacturer's restore process.
Since Pro is a "higher" level OS than Media Center, the reformat and install
is necessary. Media Center will not "overinstall" a Pro version.
If you purchased the system from a "mom & pop" system builder, you should
contact them for reinstall instructions. If the system was from a large
"reputable" manufacturer, there will be instructions somewhere in the
original documentation on how to reinstall to factory new condition.