I had a problem after a clean reinstall of XP Media Centre edition - could not find tuner. This was solved by ensuring all .NET Framework files were up to date.(Latest is .NET Framework 3.3 SP3)
I had a further problem after update to SP3 for XP - had to uninstall it, then download it to desktop, then reinstall in safe mode (I think antivirus or firewall interfered in some way. Happuage tech support on the phone were good.and they also sent a patch by e mail.
I will try to find links, but suggest you look at the above.