While browsing the PlanetAMD64 forums, I came across a thread where someone had ported open source BT8x8 drivers to Windows XP x64 Edition.
I used the 64-bit driver file to update my STB TV PCI drivers. Even though I had an issue with DScaler giving me an error under Windows XP x64, the BT8x8 driver seemed to work just fine, and I was able to watch TV!
You can download the 64-bit STB TV PCI drivers from my drivers page HERE.