This is a long shot but have you tried disabling any onboard sound devices in the bios?
I've had hit-and-miss problems that were solved that way.
I know the the 2496 control panel changed one time when I updated drivers; can't remember, but it caused me some kind of problem for a time.
I have that 2496 card in another machine running xp, but I did run it in this 64bit machine for a while without problems; even the spdif worked.
I do know that over time, certain drivers were kinda squirrely. One time I had to go through the routine of removing the drivers, device, rebooting, and installing other drivers.
(Have had to do that with video card drivers some times.)