There are two ways of doing this. I can't remember which one I used.
The first is to select "DIO2448.INF" in the driver directory, right click, and choose install.
The second is to go to the "Device Manager". (If you can't find it, do Start -> Run then type "compmgmt.msc". The Device Manager should be in the tree of options.) Find the device that currently represents the sound card. It'll be a red X if no drivers are installed or something for the Envy24. If it's something for the Envy24, right click, choose properties. Select the driver tab. Click "Uninstall." Reboot. When the system asks you to select a driver, choose "Install from a specific location." Choose "Don't search. I'll choose the driver to install." UNCHECK "Show compatible hardware." Choose "Have Disk" and select the driver directory. (I don't think this works if you don't uninstall the old driver first as I suggest; Windows will refuse to recognize the other drivers.)
If neither of these work, try double-clicking on "DIO21951.EXE" in the driver directory. This is a self-extracting archive with a slightly different copy of DIO2448.INF.