Most set top boxes work, i have a philips set top box i got from argos for around £20.
Hooking it up couldnt be easier, first buy a scart lead like this one,
Then connect the scart end to the set top box and the other three phono connectors to your WinTV card,
Next Plug in your I/R device into your usb socket on your pc and connect the little I/R sensor to the set top box (it should have a sticky side, stick it on the set top box where the I/R signal is received).
Next go into media centre and set up the remote control so that media centre can use the media centre remote for the freeview box, also set up the channels ect...
Oh and yes I/R means infra red.
Try this set top box, it should work no problem with media centre.