Can you use the nvidia codec even if you have and ati graphics card? I think i am getting myself confused here.
Initially I installed the Dvix player and codecs to play xvid/dvix files but i got the odd lockup when playing the HDTV rips of Rome. So after searching on here I installed the koepi codec. i did an uninstall for dvix first, although i am not entirely sure that this removed the codecs. I don't know if this has improved things.
From what you said I assume that ffdshow will do the same job.
TBH the picture quality on DVD's is pretty good with PowerDVD, but you know how it is - once you get everything working fine the temptation to fiddle and improve is irresistable
Paul