well, if you had SLI running previously on another version of windows, then it has to be the drivers
but i recall there needing to be the same core, and bus speeds - without having themodel #s of your cards, I guess I really can't say they won't work
SLI's requirements have relaxed a lot, you now just need the same card (eg 2x GTX470) the drivers/cards now disable any extra VRAM (gpu memory) and clock down all cards to the same clocks as the slowest that is in the machine.
The XFX card would disable 1GB of its 2GB VRAM so it has equal memory to the ASUS card, and the ASUS card would clock down to the XFX card's speeds so they were equal. By default a slower card will never step up to meet faster cards as it may be unstable at those clocks, but you can overclock them all using a 3rd party overclocking facility EG RivaTuner or MSI Afterburner (You dont have to have everything MSI for that to work)
I am using the example of GTX470 cards from ASUS and XFX because that is what happens with my system. I am unsure what the clock speeds are like on 460's but i should expect it would be a similar story good luck. I also found the ability to enable software SLI with two GT210's (multi display HTPC) i am using the latest 301.42 WHQL drivers from NVIDIAs GeForce site (www.geforce.com)