I'm not familiar with the WMC PCs, but I do have experience with HTPCs. I've got an ATI 9800 Pro AIW, which comes with an RF remote and does all the functions you could ever possibly need. Including a full-screen interface navigatable with only the remote, so it's like you aren't even using a PC.
Mine's setup as a TV, Digital VCR, DVD player/recorder, FM radio player/recorder, and game system.
edit:
You can buy external USB TV tuners, but only for USB 2.0.
www.Newegg.com has plenty as well as the ATI cards.