Frodo:
The 61.77s are listed at an Nvidia help/support page as the best for Win XP, they were developed to address some display issues with Nvidia cards, that is before the GeForce 6600 cards came on the market.
I have a 6600 GT but had to remove it from my system; I need a more pwerful PS which I was not aware of when I bought the card. I am now back to using my old GeForce 5200 card til I can get a PS upgrade.