Why download the sound driver for presario c791tu for operating system window xp

The only reason why would be to gain the full advantage of the Sound Cards capibility. If the Sound quality is fine.. no need to change it.
They do upgrades and patches over time to enhance the proformance...consider doing the upgrade. You can roll it back if you don't like it or it won't work. Generaly speaking you should see the upgrade from the Microsoft Upgrade Site... I would install all of the upgrades MS recommends not just the critical ones. Again... you can always roll back to the old drivers.