When the Leadtech card is still in there, delete the drivers, then shut the PC down.
Insert the Asus card (I have the exact same model) then boot up, and XP should find the card.
However, do not install any drivers quite yet.
Make sure you have
the Detonator ones from Nvidia, they are the best to use.
Do not use the cheesy drivers from the vid card makers themselves.
The Nvidia ones are way better.
Just download them first, then after you go through all of the above, execute the file you DL'd after XP boots up.
By the way, does your 9520 have the fan on it, or the new heat sink?
My fan crapped out after only a few months.
See
this post.