EDIT: What are the KWorld tuners like? I see you're using one of their USB models. At least one of the PCI models has passthrough and they're pretty cheap as well. Presumably as long as they have BDA drivers they'll work with MP.
I've had no problems with the Kworld 399U I use (on Vista), although I suggested it to someone running XP recently and they had a frustrating time with it - https://forum.team-mediaportal.com/tv-cards-333/dvb-t-card-anyone-72869/
Tony