You have it backwords
All HDTV tuner cards need no encoder since the digital ATSC broadcasts are
already encoded.
ATI introduced a software encoder with MCE 2004 for its AIW N TSC tuner
cards which was approved by Microsoft as long as a HT or dual CPU system was
used due to the high CPU utilization required for software encoding.
With the introduction of multiple tuner support by MS with the introduction
of MCE2005 MS would no longer approved software encoders in order for a
vendor to qualify their cards for the MCE2005 logo program. ATI of course
supports this implementation on their own.
In early summer of 2005 ATI upgraded the software encoder that worked with
MCE 2004 so it would work with MCE 2005 and provided it to AIW card
purcharsers as part of the ATI MMC media center application. AFAIK ATI has
not upgraded this software encoder again for VISTA so that it can be used
with AIW cards or with the NTSC tuner on the HDTV tuner card.
Since Vista does not require an NTSC tuner card in order for a ATSC tuner
card to be used the ATSC tuner side of the HDTV wonder should work fine with
vista since as I stated before all ATSC video is already encoded so no
encoding hardware or software is required.