You will need a USB IR-receiver and software so that your computer can receive the harmony's signals and translate that into computer commands.
I don't have any recommendations at the moment, never dealt with this kind of setup (most people just get an MCE remote, which includes the required hardware/software if applicable)
It's probably easiest/cheapest to just get an MCE remote package.
Just search at any electronics/computer retailer for "MCE remote" and you should get lots of hits.
IR remotes are line of sight only, where as RF remotes give you more flexibility as they can go through objects (often more expensive, though)
It all depends on how you define Media Center. Most would also include a TV tuner in the mix so that you can also have TV on your PC. You can buy a TV tuner bundled with the MCE remote, and sometimes it is on sale for less than $50. If your PC is running Vista HP (or better) or XP MCE then you have the software that you need.
If you want to learn more about Media Center PCs, or Home Theatre PCs (HTPCs) check out my story posted in this thread.