Although SP 3 has been known to create problems with XP/MCE 2005, I can't answer
the question of whether this is what's causing your specific problem.
What I can suggest is that you search the Hauppague website for the latest XP MEDIA
CENTER 2005 drivers for your particular hardware, then read through & follow any/all
installation notes.
It also sounds like you will need to run the MCE tuner set-up again and add the ATSC
tuners to the list of attached tuners (when in set-up, you will see some check boxes to
specify which tuner(s) you are setting up)
BTW, the reason you did not see any references to SP 2 when you ran Windows update
is, SP 3 is a complete rollup of all previous MS updates and patches.