If most of your stations are in one direction, get a directional HD antenna and an amplifier, I like electroline, check out ebay, get a new one if you can. For cable, if all your interested in is the OTA channels, you can get the base OTA package (usually like $15/month) which is ridiculous compared to investing in a good antenna. (And you need to record QM channels, and hope vista doesn't bloc them) If you want other cable channels, you won't be able to record HD, unless you invest $250 in the HD PVR. Then you probably need to upgrade your htpc for H.264 HD video.
Software is also an issue, I wouldn't recommend MCE for my worst enemy (let alone the vista version). You're better off with Beyondtv or Sage, or if you tinker Media Portal. You'll get better pvr support from these programs. Otherwise, I recommend just using the Wintv software that came with your Hauppauge card. Hauppauge also has a decent signal monitor that will let you know how well you can tune in stations.