Figured it out. Windows Media Center expects a debounced-type IR remote (when you press the same button twice the remote alternates the code is sends). You can disable debouncing in Windows by making a change to the Windows Registry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HidIr\Remotes\745a17a0-74d3-11d0-b6fe-00a0c90f57da]

Look for a DWORD key called "EnableDebounce" - it's likley set to 1. Set it to 0 to disable debouncing, reboot Windows, and use your HTM remote. Worked for me.

Kris