You need an IR receiver. You can buy them in Kits with selected Media Center Remotes. Also doing a Google search for USB IR receiver turns up quite a few results.
You will need to look out for driver
support for your OS and Media Center integration. Pinnacle has a kit for around $30 that will work for Media Center. It includes a remote, but you will hard pressed to find a receiver by itself for much less that has good driver support. Pinnacle Remote Kit for Windows Media Center Remote control - Infrared
I use the receiver that shipped with my Hauppauge 2250 tuner card. If you still need a tuner, that might be a good way to go.